netdev - Re: [PATCH] Enhance AF_PACKET implementation to not require high order contiguous memory allocation

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Message-ID: <1288045856.3296.19.camel@edumazet-laptop>
Date:	Tue, 26 Oct 2010 00:30:56 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"nhorman@...driver.com" <nhorman@...driver.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, jpirko@...hat.com
Subject: Re: [PATCH] Enhance AF_PACKET implementation to not require high
 order contiguous memory allocation

Le lundi 25 octobre 2010 à 18:14 -0400, nhorman@...driver.com a écrit :
> I think I remember those changes and IIrc yes,  tcpdump will make
> several attempts to get buffers of an appropriate size.  But while it
> tries to do that it bogs the system trying to write out pagecahe,
> swap, etc.  And that activity doesn't guarantee success.  His does
> either, but getting 5 order 0 pages is far easier and less intrusive
> to a loaded system than trying to get 1 order 4 chunk.  That's all I'm
> trying to accomplish here.  Just making it easier to use af_packet
> sockets without interfering with system performance
> 

Actually, using vmalloc() would probably hurt performance, because of
extra TLB pressure.

Of course, on recent x86 hardware you dont notice that much...

If not, why af_packet would use such convoluted double array of
'compound pages' ?

Also, on x86_32, vmalloc()/vmap() space is small (128 MB) so you might
exhaust it pretty fast with several sniffers running.

I would try a two level thing : Try to get high order pages, and
fallback on low order pages, but normally libpcap does this for us ?



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