Re: [PATCH] mm/slub: don't wait for high-order page allocation

[Michal Hocko]

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.

> 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 <iamjoonsoo.kim@xxxxxxx>
> ---
>  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
> 
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-- 
Michal Hocko
SUSE Labs
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