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BUG/MEDIUM: pool: try once to allocate from another bucket if empty

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In order to limit inter-thread contention on the global pool, in 2.9-dev3
with commit 7bf829ace ("MAJOR: pools: move the shared pool's free_list
over multiple buckets"), it was decided that if the selected bucket had
an empty free list, we would simply give up and fall back to the OS
allocator.

But this causes allocations to be made from the OS for certain threads,
to be released to overloaded pools that are sent back to the OS. One
visible effect is that sending a lot of traffic using h2load with 100
parallel streams over 100 connections causes 5-10k buffers to be
allocated, then reducing the load to only 10 connections doesn't make
these allocations go down, just because some buckets are no longer
visited.

Tests show that giving a second chance to pick another bucket in this
case is sufficient to visit all other buckets and recycle their pending
objects. Now "show pools" that starts at 10k buffers at 100 connections
goes down to about 150 with 1 connection and 100 streams in a fraction
of a second.

No backport is needed, as the issue is only in 2.9.
This commit is contained in:
Willy Tarreau 2023-11-08 16:44:20 +01:00
parent a9ae094b27
commit a57f2a5cfe
1 changed files with 7 additions and 2 deletions
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9
src/pool.c
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@ -672,9 +672,14 @@ void pool_refill_local_from_shared(struct pool_head *pool, struct pool_cache_hea
bucket = pool_tbucket();
ret = _HA_ATOMIC_LOAD(&pool->buckets[bucket].free_list);
count = 0;
do {
/* look for an apparently non-busy entry */
while (unlikely(ret == POOL_BUSY)) {
/* look for an apparently non-busy entry. If we hit a busy pool
* we retry with another random bucket. And if we encounter a
* NULL, we retry once with another random bucket. This is in
* order to prevent object accumulation in other buckets.
*/
while (unlikely(ret == POOL_BUSY || (ret == NULL && count++ < 1))) {
bucket = statistical_prng() % CONFIG_HAP_POOL_BUCKETS;
ret = _HA_ATOMIC_LOAD(&pool->buckets[bucket].free_list);
}