Re: [PATCH] mm/slub: don't wait for high-order page allocation
From: Michal Hocko <mhocko@kernel.org>
Date: 2015-08-04 13:15:30
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lkml
On Fri 31-07-15 10:09:50, Joonsoo Kim wrote:
Almost description is copied from commit fb05e7a89f50
("net: don't wait for order-3 page allocation").
I saw excessive direct memory reclaim/compaction triggered by slub.
This causes performance issues and add latency. Slub uses high-order
allocation to reduce internal fragmentation and management overhead. But,
direct memory reclaim/compaction has high overhead and the benefit of
high-order allocation can't compensate the overhead of both work.
This patch makes auxiliary high-order allocation atomic. If there is
no memory pressure and memory isn't fragmented, the alloction will still
success, so we don't sacrifice high-order allocation's benefit here.But you are also giving those allocations access to a portion of the memory reserves which doesn't sound like an intenteded behavior here. At least the changelog doesn't imply anything like that. I am not oppposed to your patch but I think we should do something about the !__GFP_WAIT behavior. This is too subtle and the mere fact the caller doesn't want or cannot sleep doesn't make it a reserve consumer automatically. We have __GFP_HIGH for that purpose. If this is not desirable because of the regression risk then we might need a new gfp flag for a best effort allocation which will fail in case we have to dive into costly reclaim.
quoted hunk ↗ jump to hunk
If the atomic allocation fails, direct memory reclaim/compaction will not be triggered, allocation fallback to low-order immediately, hence the direct memory reclaim/compaction overhead is avoided. In the allocation failure case, kswapd is waken up and trying to make high-order freepages, so allocation could success next time. Following is the test to measure effect of this patch. System: QEMU, CPU 8, 512 MB Mem: 25% memory is allocated at random position to make fragmentation. Memory-hogger occupies 150 MB memory. Workload: hackbench -g 20 -l 1000 Average result by 10 runs (Base va Patched) elapsed_time(s): 4.3468 vs 2.9838 compact_stall: 461.7 vs 73.6 pgmigrate_success: 28315.9 vs 7256.1 Signed-off-by: Joonsoo Kim <redacted> --- mm/slub.c | 2 ++ 1 file changed, 2 insertions(+)diff --git a/mm/slub.c b/mm/slub.c index 257283f..2d02a36 100644 --- a/mm/slub.c +++ b/mm/slub.c@@ -1364,6 +1364,8 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node) * so we fall-back to the minimum order allocation. */ alloc_gfp = (flags | __GFP_NOWARN | __GFP_NORETRY) & ~__GFP_NOFAIL; + if ((alloc_gfp & __GFP_WAIT) && oo_order(oo) > oo_order(s->min)) + alloc_gfp = alloc_gfp & ~__GFP_WAIT; page = alloc_slab_page(s, alloc_gfp, node, oo); if (unlikely(!page)) {-- 1.9.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
-- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>