netdev - Re: [net-next PATCH 1/6] net: Split netdev_alloc_frag into __alloc_page_frag and add __napi_alloc

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Date:	Wed, 10 Dec 2014 08:02:08 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...hat.com>
Cc:	netdev@...r.kernel.org, ast@...mgrid.com, davem@...emloft.net,
	brouer@...hat.com
Subject: Re: [net-next PATCH 1/6] net: Split netdev_alloc_frag into
 __alloc_page_frag and add __napi_alloc_frag

On Tue, 2014-12-09 at 19:40 -0800, Alexander Duyck wrote:

> I also took the opportunity to refactor the core bits that were placed in
> __alloc_page_frag.  First I updated the allocation to do either a 32K
> allocation or an order 0 page.  This is based on the changes in commmit
> d9b2938aa where it was found that latencies could be reduced in case of
> failures. 


GFP_KERNEL and GFP_ATOMIC allocation constraints are quite different.

I have no idea how expensive it is to attempt order-3, order-2, order-1
allocations with GFP_ATOMIC.

I did an interesting experiment on mlx4 driver, allocating the pages
needed to store the fragments, using a small layer before the
alloc_page() that is normally used :

- Attempt order-9 allocations, and use split_page() to give the
individual pages.

Boost in performance is 10% on TCP bulk receive, because of less TLB
misses.

With huge amount of memory these days, alloc_page() tend to give pages
spread all over memory, with poor TLB locality.

With this strategy, a 1024 RX ring is backed by 2 huge pages only.


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