xfs: hard limit background CIL push size
cover xfs: hard limit background CIL push size
Hi folks,
As we've been discussing from the recent generic/530 hangs, the CIL push work can get held off for an arbitrary amount of time and result in the CIL checkpoint growing large enough that it can consume the entire log. This is not allowed - the log recovery algorithm for finding the head and tail of the log requires two checkpoints in the log so it can find the last full checkpoint that it needs to recover. That puts a hard limit on the size of the CIL checkpoints of just under 50% of the log.
The CIL currently doesn't have any enforcement on that - log space hangs due to CIL overruns are not something we've see in test or production systems until the recent unlinked list modifications we made. Hence this has laregely been a theoretical problem rather than a practical problem up to this point.
While we've made changes that avoid the CIL hold-off vectors that lead to the generic/530 hangs, we have enough data on the CIL characteristics and performance to be able to put in place hard limits without compromising performance.
The first patch limits the CIL size on large logs - we don't need to aggregate hundreds of megabytes of metadata in the CIL to realise the relogging benefits that the CIL provides. Measures show that the point at which performance starts to be affected is somewhere between 16MB and 32MB of aggregated changes in the CIL. Hence the background push threshold is limited to be 32MB on large logs, but remains at 12.5% of the log on small logs.
The second patch adds a hard limit on the CIL background push threshold. This is set to double the size of the push threshold, so at most a single CIL context will consume 25% of the log before attempts to do background pushes will block waiting for the background push of that context to start. This means that all processes that commit to a push context that is over the hard limit will sleep until the background CIL push work starts on that context. At that point, they will be woken and their next transaction commits will occur into the new CIL commit that the background push will switch over to.
This provides a hard limit on the size of CIL checkpoint of ~1/4 of the entire log, well inside the size limit that log recovery imposes on us. For maximally sized logs, this hard limit ends up being about 3% of the entire log, so it serves to keep the logged objects moving from the CIL to the AIL at a reasonable rate and spreads them over a wider range of LSNs giving more graduated (less bursty) tail pushing behaviour.
Comments welcome.
-Dave.
PATCH 1/2 xfs: Lower CIL flush limit for large logs
From: Dave Chinner <[email protected]>
The current CIL size aggregation limit is 1/8th the log size. This means for large logs we might be aggregating at least 250MB of dirty objects in memory before the CIL is flushed to the journal. With CIL shadow buffers sitting around, this means the CIL is often consuming >500MB of temporary memory that is all allocated under GFP_NOFS conditions.
Flushing the CIL can take some time to do if there is other IO ongoing, and can introduce substantial log force latency by itself. It also pins the memory until the objects are in the AIL and can be written back and reclaimed by shrinkers. Hence this threshold also tends to determine the minimum amount of memory XFS can operate in under heavy modification without triggering the OOM killer.
Modify the CIL space limit to prevent such huge amounts of pinned metadata from aggregating. We can have 2MB of log IO in flight at once, so limit aggregation to 16x this size. This threshold was chosen as it little impact on performance (on 16-way fsmark) or log traffic but pins a lot less memory on large logs especially under heavy memory pressure. An aggregation limit of 8x had 5-10% performance degradation and a 50% increase in log throughput for the same workload, so clearly that was too small for highly concurrent workloads on large logs.
This was found via trace analysis of AIL behaviour. e.g. insertion from a single CIL flush:
xfs_ail_insert: old lsn 0/0 new lsn 1/3033090 type XFS_LI_INODE flags IN_AIL
$ grep xfs_ail_insert /mnt/scratch/s.t |grep "new lsn 1/3033090" |wc -l 1721823 $
So there were 1.7 million objects inserted into the AIL from this CIL checkpoint, the first at 2323.392108, the last at 2325.667566 which was the end of the trace (i.e. it hadn't finished). Clearly a major problem.
--- fs/xfs/xfs_log_priv.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
fs/xfs/xfs_log_priv.h +2 -1
@@ -329,7 +329,8 @@ struct xfs_cil {* enforced to ensure we stay within our maximum checkpoint size bounds. * threshold, yet give us plenty of space for aggregation on large logs. */ -#define XLOG_CIL_SPACE_LIMIT (log) (log->l_logsize >> 3) +#define XLOG_CIL_SPACE_LIMIT (log) \ + min_t(int, (log)->l_logsize >> 3, BBTOB(XLOG_TOTAL_REC_SHIFT(log)) << 4)/* * ticket grant locks, queues and accounting have their own cachlines
Signature · 1 line
2.23.0.rc1
Re: [PATCH 1/2] xfs: Lower CIL flush limit for large logs
Quoted 15 lines · Dave Chinner
From: Dave Chinner <[email protected]>
The current CIL size aggregation limit is 1/8th the log size. This means for large logs we might be aggregating at least 250MB of dirty objects in memory before the CIL is flushed to the journal. With CIL shadow buffers sitting around, this means the CIL is often consuming >500MB of temporary memory that is all allocated under
GFP_NOFSconditions.Flushing the CIL can take some time to do if there is other IO ongoing, and can introduce substantial log force latency by itself. It also pins the memory until the objects are in the AIL and can be written back and reclaimed by shrinkers. Hence this threshold also tends to determine the minimum amount of memory XFS can operate in under heavy modification without triggering the OOM killer.
Quoted 9 lines
Modify the CIL space limit to prevent such huge amounts of pinned metadata from aggregating. We can have 2MB of log IO in flight at once, so limit aggregation to 16x this size. This threshold was chosen as it little impact on performance (on 16-way fsmark) or log traffic but pins a lot less memory on large logs especially under heavy memory pressure. An aggregation limit of 8x had 5-10% performance degradation and a 50% increase in log throughput for the same workload, so clearly that was too small for highly concurrent workloads on large logs.
It would be nice to capture at least some of this reasoning in the already lengthy comment preceeding the #define....
Quoted 24 lines
This was found via trace analysis of AIL behaviour. e.g. insertion from a single CIL flush:
xfs_ail_insert: old lsn 0/0 new lsn 1/3033090 typeXFS_LI_INODEflagsIN_AIL$ grep xfs_ail_insert /mnt/scratch/s.t |grep "new lsn 1/3033090" |wc -l 1721823 $So there were 1.7 million objects inserted into the AIL from this CIL checkpoint, the first at 2323.392108, the last at 2325.667566 which was the end of the trace (i.e. it hadn't finished). Clearly a major problem.
Signed-off-by Dave Chinner <[email protected]>--- fs/xfs/xfs_log_priv.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)fs/xfs/xfs_log_priv.h +0
-0
fs/xfs/xfs_log_priv.h struct xfs_cil { · line 330
Quoted 2 lines
fs/xfs/xfs_log_priv.h +0
-0@@ -329,7 +329,8 @@ struct xfs_cil {* enforced to ensure we stay within our maximum checkpoint size bounds. * threshold, yet give us plenty of space for aggregation on large logs.
...also, does XLOG_CIL_SPACE_LIMIT correspond to "a lower threshold at which background pushing is attempted", or "a separate, higher bound"? I think it's the first (????) but ... I don't know. The name made me think it was the second, but the single use of the symbol suggests the first. :)
--D
Quoted 10 lines
fs/xfs/xfs_log_priv.h +2
-1*/ -#define XLOG_CIL_SPACE_LIMIT (log) (log->l_logsize >> 3) +#define XLOG_CIL_SPACE_LIMIT (log) \ + min_t(int, (log)->l_logsize >> 3, BBTOB(XLOG_TOTAL_REC_SHIFT(log)) << 4)/* * ticket grant locks, queues and accounting have their own cachlines Signature · 1 line
2.23.0.rc1
Re: [PATCH 1/2] xfs: Lower CIL flush limit for large logs
Quoted 26 lines · Darrick J. Wong
Quoted 15 lines · Dave Chinner
From: Dave Chinner <[email protected]>
The current CIL size aggregation limit is 1/8th the log size. This means for large logs we might be aggregating at least 250MB of dirty objects in memory before the CIL is flushed to the journal. With CIL shadow buffers sitting around, this means the CIL is often consuming >500MB of temporary memory that is all allocated under
GFP_NOFSconditions.Flushing the CIL can take some time to do if there is other IO ongoing, and can introduce substantial log force latency by itself. It also pins the memory until the objects are in the AIL and can be written back and reclaimed by shrinkers. Hence this threshold also tends to determine the minimum amount of memory XFS can operate in under heavy modification without triggering the OOM killer.
Quoted 9 lines
Modify the CIL space limit to prevent such huge amounts of pinned metadata from aggregating. We can have 2MB of log IO in flight at once, so limit aggregation to 16x this size. This threshold was chosen as it little impact on performance (on 16-way fsmark) or log traffic but pins a lot less memory on large logs especially under heavy memory pressure. An aggregation limit of 8x had 5-10% performance degradation and a 50% increase in log throughput for the same workload, so clearly that was too small for highly concurrent workloads on large logs.
Quoted 2 lines
It would be nice to capture at least some of this reasoning in the already lengthy comment preceeding the #define....
A lot of it is already there, but I will revise it.
Quoted 20 lines
Quoted 20 lines
This was found via trace analysis of AIL behaviour. e.g. insertion from a single CIL flush:
xfs_ail_insert: old lsn 0/0 new lsn 1/3033090 typeXFS_LI_INODEflagsIN_AIL$ grep xfs_ail_insert /mnt/scratch/s.t |grep "new lsn 1/3033090" |wc -l 1721823 $So there were 1.7 million objects inserted into the AIL from this CIL checkpoint, the first at 2323.392108, the last at 2325.667566 which was the end of the trace (i.e. it hadn't finished). Clearly a major problem.
Signed-off-by Dave Chinner <[email protected]>--- fs/xfs/xfs_log_priv.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
Quoted 13 lines
fs/xfs/xfs_log_priv.h struct xfs_cil { · line 330
Quoted 7 lines
fs/xfs/xfs_log_priv.h +0
-0@@ -329,7 +329,8 @@ struct xfs_cil {* enforced to ensure we stay within our maximum checkpoint size bounds. * threshold, yet give us plenty of space for aggregation on large logs. ...also, does
XLOG_CIL_SPACE_LIMITcorrespond to "a lower threshold at which background pushing is attempted", or "a separate, higher bound"? I think it's the first (????) but ... I don't know. The name made me think it was the second, but the single use of the symbol suggests the first. :)
See, the comment here talks about two limits, because that was how the initial implementation worked - the background CIL push was not async, so there was some juggling done to prevent new commits from blocking on background pushes in progress unless the size was actually growing to large. This patch pretty much describes the whole issue here:
https://lore.kernel.org/linux-xfs/[email protected]/
That's in commit 80168676ebfe ("xfs: force background CIL push under sustained load") which went into 2.6.38 or so. The cause of the problem in that case was concurrent transaction commit load causing lock contention and preventing a background push from getting the context lock to do the actual push.
The hard limit in the CIL code was dropped when the background push was converted to run asynchronously to use a work queue in 2012 as it allowed the locking to be changed (down_write_trylock -> down_write) to turn it into a transaction commit barrier while the contexts are switched over. That was done in 2012 via commit 4c2d542f2e78 ("xfs: Do background CIL flushes via a workqueue") and so we haven't actually capped CIL checkpoint sizes since 2012.
Essentially, the comment you point out documents the two limits from the original code, and this commit is restoring that behaviour for background CIL pushes....
I'll do some work to update it all.
Cheers,
Dave.
Signature · 2 lines
Dave Chinner [email protected]
Re: [PATCH 1/2] xfs: Lower CIL flush limit for large logs
Quoted 73 lines · Dave Chinner
Quoted 26 lines · Darrick J. Wong
Quoted 15 lines · Dave Chinner
From: Dave Chinner <[email protected]>
The current CIL size aggregation limit is 1/8th the log size. This means for large logs we might be aggregating at least 250MB of dirty objects in memory before the CIL is flushed to the journal. With CIL shadow buffers sitting around, this means the CIL is often consuming >500MB of temporary memory that is all allocated under
GFP_NOFSconditions.Flushing the CIL can take some time to do if there is other IO ongoing, and can introduce substantial log force latency by itself. It also pins the memory until the objects are in the AIL and can be written back and reclaimed by shrinkers. Hence this threshold also tends to determine the minimum amount of memory XFS can operate in under heavy modification without triggering the OOM killer.
Quoted 9 lines
Modify the CIL space limit to prevent such huge amounts of pinned metadata from aggregating. We can have 2MB of log IO in flight at once, so limit aggregation to 16x this size. This threshold was chosen as it little impact on performance (on 16-way fsmark) or log traffic but pins a lot less memory on large logs especially under heavy memory pressure. An aggregation limit of 8x had 5-10% performance degradation and a 50% increase in log throughput for the same workload, so clearly that was too small for highly concurrent workloads on large logs.
Quoted 2 lines
It would be nice to capture at least some of this reasoning in the already lengthy comment preceeding the #define....
A lot of it is already there, but I will revise it.
Quoted 20 lines
Quoted 20 lines
This was found via trace analysis of AIL behaviour. e.g. insertion from a single CIL flush:
xfs_ail_insert: old lsn 0/0 new lsn 1/3033090 typeXFS_LI_INODEflagsIN_AIL$ grep xfs_ail_insert /mnt/scratch/s.t |grep "new lsn 1/3033090" |wc -l 1721823 $So there were 1.7 million objects inserted into the AIL from this CIL checkpoint, the first at 2323.392108, the last at 2325.667566 which was the end of the trace (i.e. it hadn't finished). Clearly a major problem.
Signed-off-by Dave Chinner <[email protected]>--- fs/xfs/xfs_log_priv.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)Quoted 13 lines
fs/xfs/xfs_log_priv.h struct xfs_cil { · line 330
Quoted 7 lines
fs/xfs/xfs_log_priv.h +0
-0@@ -329,7 +329,8 @@ struct xfs_cil {* enforced to ensure we stay within our maximum checkpoint size bounds. * threshold, yet give us plenty of space for aggregation on large logs. ...also, does
XLOG_CIL_SPACE_LIMITcorrespond to "a lower threshold at which background pushing is attempted", or "a separate, higher bound"? I think it's the first (????) but ... I don't know. The name made me think it was the second, but the single use of the symbol suggests the first. :)See, the comment here talks about two limits, because that was how the initial implementation worked - the background CIL push was not async, so there was some juggling done to prevent new commits from blocking on background pushes in progress unless the size was actually growing to large. This patch pretty much describes the whole issue here:
Quoted 8 lines
https://lore.kernel.org/linux-xfs/[email protected]/
That's in commit
80168676ebfe("xfs: force background CIL push under sustained load") which went into 2.6.38 or so. The cause of the problem in that case was concurrent transaction commit load causing lock contention and preventing a background push from getting the context lock to do the actual push.
More related to the next patch, but what prevents a similar but generally unbound concurrent workload from exceeding the new hard limit once transactions start to block post commit?
Brian
Quoted 20 lines
The hard limit in the CIL code was dropped when the background push was converted to run asynchronously to use a work queue in 2012 as it allowed the locking to be changed (
down_write_trylock->down_write) to turn it into a transaction commit barrier while the contexts are switched over. That was done in 2012 via commit4c2d542f2e78("xfs: Do background CIL flushes via a workqueue") and so we haven't actually capped CIL checkpoint sizes since 2012.Essentially, the comment you point out documents the two limits from the original code, and this commit is restoring that behaviour for background CIL pushes....
I'll do some work to update it all.
Cheers,
Dave.
Signature · 2 lines
Dave Chinner [email protected]
Re: [PATCH 1/2] xfs: Lower CIL flush limit for large logs
Quoted 11 lines · Brian Foster
Quoted 6 lines · Dave Chinner
That's in commit
80168676ebfe("xfs: force background CIL push under sustained load") which went into 2.6.38 or so. The cause of the problem in that case was concurrent transaction commit load causing lock contention and preventing a background push from getting the context lock to do the actual push.More related to the next patch, but what prevents a similar but generally unbound concurrent workload from exceeding the new hard limit once transactions start to block post commit?
The new code, like the original code, is not actually a "hard" limit. It's essentially just throttles ongoing work until the CIL push starts. In this case, it forces the current process to give up the CPU immediately once over the CIL high limit, which allows the workqueue to run the push work straight away.
I thought about making it a "hard limit" by blocking before the CIL insert, but that's no guarantee that by the time we get woken and add the new commit to the CIL that this new context has not already gone over the hard limit. i.e. we block the unbound concurrency before commit, then let it all go in a thundering herd on the new context and immeidately punch that way over the hard threshold again. To avoid this, we'd probably need a CIL ticket and grant mechanism to make CIL insertion FIFO and wakeups limited by remaining space in the CIL. I'm not sure we actually need such a complex solution, especially considering the potential serialisation problems it introduces in what is a highly concurrent fast path...
Cheers,
Dave.
Signature · 2 lines
Dave Chinner [email protected]
Re: [PATCH 1/2] xfs: Lower CIL flush limit for large logs
Quoted 19 lines · Dave Chinner
Quoted 11 lines · Brian Foster
Quoted 6 lines · Dave Chinner
That's in commit
80168676ebfe("xfs: force background CIL push under sustained load") which went into 2.6.38 or so. The cause of the problem in that case was concurrent transaction commit load causing lock contention and preventing a background push from getting the context lock to do the actual push.More related to the next patch, but what prevents a similar but generally unbound concurrent workload from exceeding the new hard limit once transactions start to block post commit?
The new code, like the original code, is not actually a "hard" limit. It's essentially just throttles ongoing work until the CIL push starts. In this case, it forces the current process to give up the CPU immediately once over the CIL high limit, which allows the workqueue to run the push work straight away.
Quoted 12 lines
I thought about making it a "hard limit" by blocking before the CIL insert, but that's no guarantee that by the time we get woken and add the new commit to the CIL that this new context has not already gone over the hard limit. i.e. we block the unbound concurrency before commit, then let it all go in a thundering herd on the new context and immeidately punch that way over the hard threshold again. To avoid this, we'd probably need a CIL ticket and grant mechanism to make CIL insertion FIFO and wakeups limited by remaining space in the CIL. I'm not sure we actually need such a complex solution, especially considering the potential serialisation problems it introduces in what is a highly concurrent fast path...
Ok. The latter is more of what I'd expect to see if the objective is truly to hard cap CIL size, FWIW. This seems more reasonable if the objective is to yield committers under overloaded CIL conditions, with the caveat that CIL size is still not hard capped, so long as the documentation and whatnot is updated to more accurately reflect the implementation (and at a glance, it looks like that has happened in the next version..).
Brian
Quoted 6 lines
Cheers,
Dave.
Signature · 2 lines
Dave Chinner [email protected]
PATCH 2/2 xfs: hard limit the background CIL push
In reply to Dave ChinnerFrom: Dave Chinner <[email protected]>
In certain situations the background CIL push can be indefinitely delayed. While we have workarounds from the obvious cases now, it doesn't solve the underlying issue. This issue is that there is no upper limit on the CIL where we will either force or wait for a background push to start, hence allowing the CIL to grow without bound until it consumes all log space.
To fix this, add a new wait queue to the CIL which allows background pushes to wait for the CIL context to be switched out. This happens when the push starts, so it will allow us to block incoming transaction commit completion until the push has started. This will only affect processes that are running modifications, and only when the CIL threshold has been significantly overrun.
This has no apparent impact on performance, and doesn't even trigger until over 45 million inodes had been created in a 16-way fsmark test on a 2GB log. That was limiting at 64MB of log space used, so the active CIL size is only about 3% of the total log in that case. The concurrent removal of those files did not trigger the background sleep at all.
--- fs/xfs/xfs_log_cil.c | 30 +++++++++++++++++++++++++----- fs/xfs/xfs_log_priv.h | 1 + 2 files changed, 26 insertions(+), 5 deletions(-)
fs/xfs/xfs_log_cil.c +25 -5
@@ -670,6 +670,11 @@ xlog_cil_push( push_seq = cil->xc_push_seq;ASSERT (push_seq <= ctx->sequence); +/* +* Wake up any background push waiters now this context is being pushed. +*/ +wake_up_all (&ctx->push_wait); +/* * Check if we've anything to push. If there is nothing, then we don't * move on to a new sequence number and so we have to be able to push
@@ -746,6 +751,7 @@ xlog_cil_push(*/ INIT_LIST_HEAD (&new_ctx->committing);INIT_LIST_HEAD (&new_ctx->busy_extents); +init_waitqueue_head (&new_ctx->push_wait); new_ctx->sequence = ctx->sequence +1 ; new_ctx->cil = cil; cil->xc_ctx = new_ctx;
@@ -898,7 +904,7 @@ xlog_cil_push_work(* checkpoint), but commit latency and memory usage limit this to a smaller * size. */ -static void +static bool xlog_cil_push_background (struct xlog *log) {
@@ -915,14 +921,28 @@ xlog_cil_push_background(* space available yet. */ if (cil->xc_ctx->space_used <XLOG_CIL_SPACE_LIMIT (log)) -return ; +return false;spin_lock (&cil->xc_push_lock);if (cil->xc_push_seq < cil->xc_current_sequence) { cil->xc_push_seq = cil->xc_current_sequence;queue_work (log->l_mp->m_cil_workqueue, &cil->xc_push_work); } + +/* +* If we are well over the space limit, throttle the work that is being +* done until the push work on this context has begun. This will prevent +* the CIL from violating maximum transaction size limits if the CIL +* push is delayed for some reason. +*/ +if (cil->xc_ctx->space_used >XLOG_CIL_SPACE_LIMIT (log) *2 ) { +up_read (&cil->xc_ctx_lock); +trace_printk ("CIL space used %d" , cil->xc_ctx->space_used); +xlog_wait (&cil->xc_ctx->push_wait, &cil->xc_push_lock); +return true; + }spin_unlock (&cil->xc_push_lock); +return false; }
@@ -1038,9 +1058,8 @@ xfs_log_commit_cil(if (lip->li_ops->iop_committing) lip->li_ops->iop_committing (lip, xc_commit_lsn); } -xlog_cil_push_background (log); - -up_read (&cil->xc_ctx_lock); +if (!xlog_cil_push_background (log)) +up_read (&cil->xc_ctx_lock); }/*
@@ -1199,6 +1218,7 @@ xlog_cil_init(INIT_LIST_HEAD (&ctx->committing);INIT_LIST_HEAD (&ctx->busy_extents); +init_waitqueue_head (&ctx->push_wait); ctx->sequence =1 ; ctx->cil = cil; cil->xc_ctx = ctx;
fs/xfs/xfs_log_priv.h +1 -0
@@ -247,6 +247,7 @@ struct xfs_cil_ctx {struct xfs_log_vec *lv_chain;/* logvecs being pushed */ struct list_head iclog_entry;struct list_head committing;/* ctx committing list */ +wait_queue_head_t push_wait;/* background push throttle */ struct work_struct discard_endio_work; };
Signature · 1 line
2.23.0.rc1
Re: [PATCH 2/2] xfs: hard limit the background CIL push
Quoted 74 lines · Dave Chinner
From: Dave Chinner <[email protected]>
In certain situations the background CIL push can be indefinitely delayed. While we have workarounds from the obvious cases now, it doesn't solve the underlying issue. This issue is that there is no upper limit on the CIL where we will either force or wait for a background push to start, hence allowing the CIL to grow without bound until it consumes all log space.
To fix this, add a new wait queue to the CIL which allows background pushes to wait for the CIL context to be switched out. This happens when the push starts, so it will allow us to block incoming transaction commit completion until the push has started. This will only affect processes that are running modifications, and only when the CIL threshold has been significantly overrun.
This has no apparent impact on performance, and doesn't even trigger until over 45 million inodes had been created in a 16-way fsmark test on a 2GB log. That was limiting at 64MB of log space used, so the active CIL size is only about 3% of the total log in that case. The concurrent removal of those files did not trigger the background sleep at all.
Signed-off-by Dave Chinner <[email protected]>--- fs/xfs/xfs_log_cil.c | 30 +++++++++++++++++++++++++----- fs/xfs/xfs_log_priv.h | 1 + 2 files changed, 26 insertions(+), 5 deletions(-)fs/xfs/xfs_log_cil.c +8
-2@@ -670,6 +670,11 @@ xlog_cil_push( push_seq = cil->xc_push_seq;ASSERT (push_seq <= ctx->sequence); +/* +* Wake up any background push waiters now this context is being pushed. +*/ +wake_up_all (&ctx->push_wait); +/* * Check if we've anything to push. If there is nothing, then we don't * move on to a new sequence number and so we have to be able to push @@ -746,6 +751,7 @@ xlog_cil_push(*/ INIT_LIST_HEAD (&new_ctx->committing);INIT_LIST_HEAD (&new_ctx->busy_extents); +init_waitqueue_head (&new_ctx->push_wait); new_ctx->sequence = ctx->sequence +1 ; new_ctx->cil = cil; cil->xc_ctx = new_ctx;@@ -898,7 +904,7 @@ xlog_cil_push_work(* checkpoint), but commit latency and memory usage limit this to a smaller * size. */ -static void +static bool xlog_cil_push_background (struct xlog *log) {@@ -915,14 +921,28 @@ xlog_cil_push_background(* space available yet. */ if (cil->xc_ctx->space_used <XLOG_CIL_SPACE_LIMIT (log)) -return ; +return false;spin_lock (&cil->xc_push_lock);if (cil->xc_push_seq < cil->xc_current_sequence) { cil->xc_push_seq = cil->xc_current_sequence;queue_work (log->l_mp->m_cil_workqueue, &cil->xc_push_work); }
fs/xfs/xfs_log_cil.c xlog_cil_push_background( · line 940
Quoted 10 lines
fs/xfs/xfs_log_cil.c +10
-0+ +/* +* If we are well over the space limit, throttle the work that is being +* done until the push work on this context has begun. This will prevent +* the CIL from violating maximum transaction size limits if the CIL +* push is delayed for some reason. +*/ +if (cil->xc_ctx->space_used >XLOG_CIL_SPACE_LIMIT (log) *2 ) { +up_read (&cil->xc_ctx_lock); +trace_printk ("CIL space used %d" , cil->xc_ctx->space_used);
Needs a real tracepoint before this drops RFC status.
Quoted 9 lines
fs/xfs/xfs_log_cil.c +4
-0+xlog_wait (&cil->xc_ctx->push_wait, &cil->xc_push_lock); +return true; + }spin_unlock (&cil->xc_push_lock); +return false; }
fs/xfs/xfs_log_cil.c xfs_log_commit_cil( · line 1062
Quoted 8 lines
fs/xfs/xfs_log_cil.c +2
-3@@ -1038,9 +1058,8 @@ xfs_log_commit_cil(if (lip->li_ops->iop_committing) lip->li_ops->iop_committing (lip, xc_commit_lsn); } -xlog_cil_push_background (log); - -up_read (&cil->xc_ctx_lock); +if (!xlog_cil_push_background (log)) +up_read (&cil->xc_ctx_lock);
Hmmmm... the return value here tell us if ctx_lock has been dropped. /me wonders if this would be cleaner if xlog_cil_push_background returned having called up_read...?
--D
Quoted 26 lines
fs/xfs/xfs_log_cil.c +1
-0}/* @@ -1199,6 +1218,7 @@ xlog_cil_init(INIT_LIST_HEAD (&ctx->committing);INIT_LIST_HEAD (&ctx->busy_extents); +init_waitqueue_head (&ctx->push_wait); ctx->sequence =1 ; ctx->cil = cil; cil->xc_ctx = ctx;fs/xfs/xfs_log_priv.h +1
-0@@ -247,6 +247,7 @@ struct xfs_cil_ctx {struct xfs_log_vec *lv_chain;/* logvecs being pushed */ struct list_head iclog_entry;struct list_head committing;/* ctx committing list */ +wait_queue_head_t push_wait;/* background push throttle */ struct work_struct discard_endio_work; };Signature · 1 line
2.23.0.rc1
Re: [PATCH 2/2] xfs: hard limit the background CIL push
Quoted 79 lines · Darrick J. Wong
Quoted 78 lines · Dave Chinner
From: Dave Chinner <[email protected]>
In certain situations the background CIL push can be indefinitely delayed. While we have workarounds from the obvious cases now, it doesn't solve the underlying issue. This issue is that there is no upper limit on the CIL where we will either force or wait for a background push to start, hence allowing the CIL to grow without bound until it consumes all log space.
To fix this, add a new wait queue to the CIL which allows background pushes to wait for the CIL context to be switched out. This happens when the push starts, so it will allow us to block incoming transaction commit completion until the push has started. This will only affect processes that are running modifications, and only when the CIL threshold has been significantly overrun.
This has no apparent impact on performance, and doesn't even trigger until over 45 million inodes had been created in a 16-way fsmark test on a 2GB log. That was limiting at 64MB of log space used, so the active CIL size is only about 3% of the total log in that case. The concurrent removal of those files did not trigger the background sleep at all.
Signed-off-by Dave Chinner <[email protected]>--- fs/xfs/xfs_log_cil.c | 30 +++++++++++++++++++++++++----- fs/xfs/xfs_log_priv.h | 1 + 2 files changed, 26 insertions(+), 5 deletions(-)fs/xfs/xfs_log_cil.c +12
-2@@ -670,6 +670,11 @@ xlog_cil_push( push_seq = cil->xc_push_seq;ASSERT (push_seq <= ctx->sequence); +/* +* Wake up any background push waiters now this context is being pushed. +*/ +wake_up_all (&ctx->push_wait); +/* * Check if we've anything to push. If there is nothing, then we don't * move on to a new sequence number and so we have to be able to push @@ -746,6 +751,7 @@ xlog_cil_push(*/ INIT_LIST_HEAD (&new_ctx->committing);INIT_LIST_HEAD (&new_ctx->busy_extents); +init_waitqueue_head (&new_ctx->push_wait); new_ctx->sequence = ctx->sequence +1 ; new_ctx->cil = cil; cil->xc_ctx = new_ctx;@@ -898,7 +904,7 @@ xlog_cil_push_work(* checkpoint), but commit latency and memory usage limit this to a smaller * size. */ -static void +static bool xlog_cil_push_background (struct xlog *log) {@@ -915,14 +921,28 @@ xlog_cil_push_background(* space available yet. */ if (cil->xc_ctx->space_used <XLOG_CIL_SPACE_LIMIT (log)) -return ; +return false;spin_lock (&cil->xc_push_lock);if (cil->xc_push_seq < cil->xc_current_sequence) { cil->xc_push_seq = cil->xc_current_sequence;queue_work (log->l_mp->m_cil_workqueue, &cil->xc_push_work); } + +/* +* If we are well over the space limit, throttle the work that is being +* done until the push work on this context has begun. This will prevent
Quoted 8 lines
fs/xfs/xfs_log_cil.c xlog_cil_push_background( · line 940
Quoted 6 lines
fs/xfs/xfs_log_cil.c +6
-0+* the CIL from violating maximum transaction size limits if the CIL +* push is delayed for some reason. +*/ +if (cil->xc_ctx->space_used >XLOG_CIL_SPACE_LIMIT (log) *2 ) { +up_read (&cil->xc_ctx_lock); +trace_printk ("CIL space used %d" , cil->xc_ctx->space_used);Needs a real tracepoint before this drops RFC status.
Ok, that was just debugging stuff I forgot to remove, but I can turn it into a real tracepoint if you want.
Quoted 9 lines
Quoted 9 lines
fs/xfs/xfs_log_cil.c +4
-0+xlog_wait (&cil->xc_ctx->push_wait, &cil->xc_push_lock); +return true; + }spin_unlock (&cil->xc_push_lock); +return false; }
Quoted 13 lines
fs/xfs/xfs_log_cil.c xfs_log_commit_cil( · line 1062
Quoted 9 lines
fs/xfs/xfs_log_cil.c +2
-3@@ -1038,9 +1058,8 @@ xfs_log_commit_cil(if (lip->li_ops->iop_committing) lip->li_ops->iop_committing (lip, xc_commit_lsn); } -xlog_cil_push_background (log); - -up_read (&cil->xc_ctx_lock); +if (!xlog_cil_push_background (log)) +up_read (&cil->xc_ctx_lock);Hmmmm... the return value here tell us if
ctx_lockhas been dropped. /me wonders if this would be cleaner ifxlog_cil_push_backgroundreturned having calledup_read...?
I thought about that - was on the fence about what to do. I'll change it to be unconditional.
Cheers,
Dave.
Signature · 2 lines
Dave Chinner [email protected]
Re: [PATCH 2/2] xfs: hard limit the background CIL push
Quoted 89 lines · Dave Chinner
Quoted 79 lines · Darrick J. Wong
Quoted 78 lines · Dave Chinner
From: Dave Chinner <[email protected]>
In certain situations the background CIL push can be indefinitely delayed. While we have workarounds from the obvious cases now, it doesn't solve the underlying issue. This issue is that there is no upper limit on the CIL where we will either force or wait for a background push to start, hence allowing the CIL to grow without bound until it consumes all log space.
To fix this, add a new wait queue to the CIL which allows background pushes to wait for the CIL context to be switched out. This happens when the push starts, so it will allow us to block incoming transaction commit completion until the push has started. This will only affect processes that are running modifications, and only when the CIL threshold has been significantly overrun.
This has no apparent impact on performance, and doesn't even trigger until over 45 million inodes had been created in a 16-way fsmark test on a 2GB log. That was limiting at 64MB of log space used, so the active CIL size is only about 3% of the total log in that case. The concurrent removal of those files did not trigger the background sleep at all.
Signed-off-by Dave Chinner <[email protected]>--- fs/xfs/xfs_log_cil.c | 30 +++++++++++++++++++++++++----- fs/xfs/xfs_log_priv.h | 1 + 2 files changed, 26 insertions(+), 5 deletions(-)fs/xfs/xfs_log_cil.c +12
-2@@ -670,6 +670,11 @@ xlog_cil_push( push_seq = cil->xc_push_seq;ASSERT (push_seq <= ctx->sequence); +/* +* Wake up any background push waiters now this context is being pushed. +*/ +wake_up_all (&ctx->push_wait); +/* * Check if we've anything to push. If there is nothing, then we don't * move on to a new sequence number and so we have to be able to push @@ -746,6 +751,7 @@ xlog_cil_push(*/ INIT_LIST_HEAD (&new_ctx->committing);INIT_LIST_HEAD (&new_ctx->busy_extents); +init_waitqueue_head (&new_ctx->push_wait); new_ctx->sequence = ctx->sequence +1 ; new_ctx->cil = cil; cil->xc_ctx = new_ctx;@@ -898,7 +904,7 @@ xlog_cil_push_work(* checkpoint), but commit latency and memory usage limit this to a smaller * size. */ -static void +static bool xlog_cil_push_background (struct xlog *log) {@@ -915,14 +921,28 @@ xlog_cil_push_background(* space available yet. */ if (cil->xc_ctx->space_used <XLOG_CIL_SPACE_LIMIT (log)) -return ; +return false;spin_lock (&cil->xc_push_lock);if (cil->xc_push_seq < cil->xc_current_sequence) { cil->xc_push_seq = cil->xc_current_sequence;queue_work (log->l_mp->m_cil_workqueue, &cil->xc_push_work); } + +/* +* If we are well over the space limit, throttle the work that is being +* done until the push work on this context has begun. This will prevent Quoted 8 lines
fs/xfs/xfs_log_cil.c xlog_cil_push_background( · line 940
Quoted 6 lines
fs/xfs/xfs_log_cil.c +6
-0+* the CIL from violating maximum transaction size limits if the CIL +* push is delayed for some reason. +*/ +if (cil->xc_ctx->space_used >XLOG_CIL_SPACE_LIMIT (log) *2 ) { +up_read (&cil->xc_ctx_lock); +trace_printk ("CIL space used %d" , cil->xc_ctx->space_used);Needs a real tracepoint before this drops RFC status.
Quoted 2 lines
Ok, that was just debugging stuff I forgot to remove, but I can turn it into a real tracepoint if you want.
<shrug> You could drop it too; I was just point out the trace_printk.
(For those of you following at home, trace_printk calls generate huge debugging warnings at module load time.)
Quoted 23 lines
Quoted 9 lines
Quoted 9 lines
fs/xfs/xfs_log_cil.c +4
-0+xlog_wait (&cil->xc_ctx->push_wait, &cil->xc_push_lock); +return true; + }spin_unlock (&cil->xc_push_lock); +return false; }Quoted 13 lines
fs/xfs/xfs_log_cil.c xfs_log_commit_cil( · line 1062
Quoted 9 lines
fs/xfs/xfs_log_cil.c +2
-3@@ -1038,9 +1058,8 @@ xfs_log_commit_cil(if (lip->li_ops->iop_committing) lip->li_ops->iop_committing (lip, xc_commit_lsn); } -xlog_cil_push_background (log); - -up_read (&cil->xc_ctx_lock); +if (!xlog_cil_push_background (log)) +up_read (&cil->xc_ctx_lock);Hmmmm... the return value here tell us if
ctx_lockhas been dropped. /me wonders if this would be cleaner ifxlog_cil_push_backgroundreturned having calledup_read...?
Quoted 2 lines
I thought about that - was on the fence about what to do. I'll change it to be unconditional.
<nod>
--D
Quoted 6 lines
Cheers,
Dave.
Signature · 2 lines
Dave Chinner [email protected]