jffs2: force the jffs2 GC daemon to behave a bit better
PATCH 1/1 jffs2: force the jffs2 GC daemon to behave a bit better
I've noticed some pretty poor behavior on OLPC machines after bootup, when gdm/X are starting. The GCD monopolizes the scheduler (which in turns means it gets to do more nand i/o), which results in processes taking much much longer than they should to start.
As an example, on an OLPC machine going from OFW to a usable X (via auto-login gdm) takes 2m 30s. The majority of this time is consumed by the switch into graphical mode. With this patch, we cut a full 60s off of bootup time. After bootup, things are much snappier as well.
Note that we have seen a CRC node error with this patch that causes the machine to fail to boot, but we've also seen that problem without this patch.
--- fs/jffs2/background.c | 18 +++++++++++------- 1 files changed, 11 insertions(+), 7 deletions(-)
fs/jffs2/background.c +11 -7
@@ -95,13 +95,17 @@ static int jffs2_garbage_collect_thread(void *_c)schedule (); } -/* This thread is purely an optimisation. But if it runs when -other things could be running, it actually makes things a -lot worse. Use yield() and put it at the back of the runqueue -every time. Especially during boot, pulling an inode in -with read_inode() is much preferable to having the GC thread -get there first. */ -yield (); +/* Problem - immediately after bootup, the GCD spends a lot +* of time in places like jffs2_kill_fragtree(); so much so +* that userspace processes (like gdm and X) are starved +* despite plenty of cond_resched()s and renicing. Yield() +* doesn't help, either (presumably because userspace and GCD +* are generally competing for a higher latency resource - +* disk). +* This forces the GCD to slow the hell down. Pulling an +* inode in with read_inode() is much preferable to having +* the GC thread get there first. */ +schedule_timeout_interruptible (msecs_to_jiffies (50 ));/* Put_super will send a SIGKILL and then wait on the sem. */
Signature · 1 line
1.5.6.5
Quoted 6 lines · On Sun, 2 Nov 2008 13:04:49 -0500 Andres Salomon <[email protected]> wrote:
I've noticed some pretty poor behavior on OLPC machines after bootup, when gdm/X are starting. The GCD monopolizes the scheduler (which in turns means it gets to do more nand i/o), which results in processes taking much much longer than they should to start.
Quoted 8 lines
As an example, on an OLPC machine going from OFW to a usable X (via auto-login gdm) takes 2m 30s. The majority of this time is consumed by the switch into graphical mode. With this patch, we cut a full 60s off of bootup time. After bootup, things are much snappier as well.
Note that we have seen a CRC node error with this patch that causes the machine to fail to boot, but we've also seen that problem without this patch.
Well you've observed one of the problems with yield(). The other problem is that this thread can be starved for fantastic amounts of time when the system is busy.
yield() is so unpredictable that basically any use of it should be viewed as a bug.
Quoted 23 lines
--- fs/jffs2/background.c | 18 +++++++++++------- 1 files changed, 11 insertions(+), 7 deletions(-)fs/jffs2/background.c +4
-7@@ -95,13 +95,17 @@ static int jffs2_garbage_collect_thread(void *_c)schedule (); } -/* This thread is purely an optimisation. But if it runs when -other things could be running, it actually makes things a -lot worse. Use yield() and put it at the back of the runqueue -every time. Especially during boot, pulling an inode in -with read_inode() is much preferable to having the GC thread -get there first. */ -yield (); +/* Problem - immediately after bootup, the GCD spends a lot +* of time in places like jffs2_kill_fragtree(); so much so +* that userspace processes (like gdm and X) are starved +* despite plenty of cond_resched()s and renicing. Yield()
fs/jffs2/background.c static int jffs2_garbage_collect_thread(void *_c) · line 111
Quoted 10 lines
fs/jffs2/background.c +7
-0+* doesn't help, either (presumably because userspace and GCD +* are generally competing for a higher latency resource - +* disk). +* This forces the GCD to slow the hell down. Pulling an +* inode in with read_inode() is much preferable to having +* the GC thread get there first. */ +schedule_timeout_interruptible (msecs_to_jiffies (50 ));/* Put_super will send a SIGKILL and then wait on the sem. */
Yeah. It doesn't matter much - almost any change you can make in there will improve the current code.
David, I do think we should fix this in 2.6.28 (at least).
Perhaps this is an application for SCHED_IDLE?