netdev - Re: [PATCH net-next 1/3] net: allow > 0 order atomic page alloc in skb_page_frag

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Date:	Thu, 02 Jan 2014 16:56:31 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Debabrata Banerjee <dbavatar@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, mwdalton@...gle.com,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	rusty@...tcorp.com.au, mst@...hat.com, jasowang@...hat.com,
	virtualization@...ts.linux-foundation.org,
	"Banerjee, Debabrata" <dbanerje@...mai.com>, jbaron@...mai.com,
	Joshua Hunt <johunt@...mai.com>
Subject: Re: [PATCH net-next 1/3] net: allow > 0 order atomic page alloc in
 skb_page_frag_refill

On Thu, 2014-01-02 at 19:42 -0500, Debabrata Banerjee wrote:
> Currently because of how mm behaves (3.10.y) the code even before the
> patch is a problem. I believe what may fix it is if instead of just
> removing the conditional on __GFP_WAIT, the initial order > 0
> allocation should be made GFP_ATOMIC, then fallback to the original
> gfp mask for the order-0 allocations.
> 
> On systems that have highly fragmented main memory with pressure,
> skb_page_frag_refill() causes problems. mm enters significant
> compaction cycles on all cpu's which in itself is bad (add
> considerable spinlock contention in isolate_migratepages_range() for
> several seconds in kernel at 100% cpu), however even without this
> happening basically we have large memory reclaimation when only
> allocations from order-3 were necessary. For example, I might see half
> the existing page cache on a system (2GB out of 8GB) reclaimed in a
> burst, which effectively means the application has to wait even longer
> after this compact/reclaim cycle for those pages to be read back from
> disk. This is a significant reduction in useful memory from before
> skb_page_frag_refill() existed, as one of our systems could run in
> steady state will little free memory and 100% fragmentation. Now I see
> 10-30x more memory free (read: not utilized). Order > 0 allocations
> were happening rarely before, now it happens consistently from this
> function.
> 
> My suggestion above would avoid mm going through
> __alloc_pages_direct_compact() and triggering the bad events above. It
> will take me several days to try this experiment.

My suggestion is to use a recent kernel, and/or eventually backport the
mm fixes if any.

order-3 allocations should not reclaim 2GB out of 8GB.

There is a reason PAGE_ALLOC_COSTLY_ORDER exists and is 3



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