New: default values of /proc/sys/net/ipv4/udp_mem does not consider huge page allocatio

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Yes, we've made similar mistakes in other places.

I don't think we really have an official formula for what callers should be doing here. net/ipv4/udp.c:udp_init() does

        nr_pages = totalram_pages - totalhigh_pages;                            

which assumes that totalram_pages does not include the pages which were lost to hugepage allocations.

I *think* that this is now the case, but it wasn't always the case - we made relatively recent fixes to the totalram_pages maintenance.

Perhaps UDP should be using the misnamed nr_free_buffer_pages() here.

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For anyone who may not have read the bugzilla, a possibly larger concern subsequently discovered is that actual kernel memory consumption is double the total of the values reported by 'netstat -nau', at least when mostly small packets are received and a RHEL 5 kernel is in use. The tunable enforces based on the 'netstat' value rather than the actual value in the RH kernel. Maybe not an issue in the mainline, but it took a few additional system hangs in the lab before we figured this out and divided the 'udm_mem' maximum value in half.

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Le mercredi 06 juillet 2011 à 21:31 -0400, [email protected] a écrit :

Several problems here

1) Hugepages can be setup after system boot, and udp_mem/tcp_mem not updated accordingly.

2) Using SLUB debug or kmemcheck for instance adds lot of overhead, that we dont take into account (it would probably be expensive to do so). Even ksize(ptr) is not able to really report memory usage of an object.

3) What happens if both tcp and udp sockets are in use on the system, shouls we half both udp_mem and tcp_mem just in case ?

Now if you also use SCTP sockets, UDP-Lite sockets, lot of file mappings, huge pages, conntracking, posix timers (currently not limited), threads, ..., what happens ? Should we then set udp_mem to 1% of current limit just in case ?

What about fixing the real problem instead ?

When you say the system freezes, is it in the UDP stack, or elsewhere ?

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Lets use following patch ?

[PATCH] net: refine {udp|tcp|sctp}_mem limits

Current tcp/udp/sctp global memory limits are not taking into account hugepages allocations, and allow 50% of ram to be used by buffers of a single protocol [ not counting space used by sockets / inodes ...]

Lets use nr_free_buffer_pages() and allow a default of 1/8 of kernel ram per protocol, and a minimum of 128 pages.
Heavy duty machines sysadmins probably need to tweak limits anyway.

References: https://bugzilla.stlinux.com/show_bug.cgi?id=38032

---
 net/ipv4/tcp.c      |   10 ++--------
 net/ipv4/udp.c      |   10 ++--------
 net/sctp/protocol.c |   11 +----------
 3 files changed, 5 insertions(+), 26 deletions(-)

index 054a59d..46febca 100644
@@ -3220,7 +3220,7 @@ __setup("thash_entries=", set_thash_entries);
 void __init tcp_init(void)
 {
 	struct sk_buff *skb = NULL;
-	unsigned long nr_pages, limit;
+	unsigned long limit;
 	int i, max_share, cnt;
 	unsigned long jiffy = jiffies;
 

@@ -3277,13 +3277,7 @@ void __init tcp_init(void)
 	sysctl_tcp_max_orphans = cnt / 2;
 	sysctl_max_syn_backlog = max(128, cnt / 256);
 
-	/* Set the pressure threshold to be a fraction of global memory that
-	 * is up to 1/2 at 256 MB, decreasing toward zero with the amount of
-	 * memory, with a floor of 128 pages.
-	 */
-	nr_pages = totalram_pages - totalhigh_pages;
-	limit = min(nr_pages, 1UL<<(28-PAGE_SHIFT)) >> (20-PAGE_SHIFT);
-	limit = (limit * (nr_pages >> (20-PAGE_SHIFT))) >> (PAGE_SHIFT-11);
+	limit = nr_free_buffer_pages() / 8;
 	limit = max(limit, 128UL);
 	sysctl_tcp_mem[0] = limit / 4 * 3;
 	sysctl_tcp_mem[1] = limit;

index 48cd88e..198f75b 100644
@@ -2209,16 +2209,10 @@ void __init udp_table_init(struct udp_table *table, const char *name)
 
 void __init udp_init(void)
 {
-	unsigned long nr_pages, limit;
+	unsigned long limit;
 
 	udp_table_init(&udp_table, "UDP");
-	/* Set the pressure threshold up by the same strategy of TCP. It is a
-	 * fraction of global memory that is up to 1/2 at 256 MB, decreasing
-	 * toward zero with the amount of memory, with a floor of 128 pages.
-	 */
-	nr_pages = totalram_pages - totalhigh_pages;
-	limit = min(nr_pages, 1UL<<(28-PAGE_SHIFT)) >> (20-PAGE_SHIFT);
-	limit = (limit * (nr_pages >> (20-PAGE_SHIFT))) >> (PAGE_SHIFT-11);
+	limit = nr_free_buffer_pages() / 8;
 	limit = max(limit, 128UL);
 	sysctl_udp_mem[0] = limit / 4 * 3;
 	sysctl_udp_mem[1] = limit;

index 67380a2..207175b 100644
@@ -1058,7 +1058,6 @@ SCTP_STATIC __init int sctp_init(void)
 	int status = -EINVAL;
 	unsigned long goal;
 	unsigned long limit;
-	unsigned long nr_pages;
 	int max_share;
 	int order;
 

@@ -1148,15 +1147,7 @@ SCTP_STATIC __init int sctp_init(void)
 	/* Initialize handle used for association ids. */
 	idr_init(&sctp_assocs_id);
 
-	/* Set the pressure threshold to be a fraction of global memory that
-	 * is up to 1/2 at 256 MB, decreasing toward zero with the amount of
-	 * memory, with a floor of 128 pages.
-	 * Note this initializes the data in sctpv6_prot too
-	 * Unabashedly stolen from tcp_init
-	 */
-	nr_pages = totalram_pages - totalhigh_pages;
-	limit = min(nr_pages, 1UL<<(28-PAGE_SHIFT)) >> (20-PAGE_SHIFT);
-	limit = (limit * (nr_pages >> (20-PAGE_SHIFT))) >> (PAGE_SHIFT-11);
+	limit = nr_free_buffer_pages() / 8;
 	limit = max(limit, 128UL);
 	sysctl_sctp_mem[0] = limit / 4 * 3;
 	sysctl_sctp_mem[1] = limit;

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From: Eric Dumazet <[email protected]>
Date: Thu, 07 Jul 2011 06:38:10 +0200

Applied.

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