netdev - Re: [PATCH net-next-2.6] net: speedup udp receive path

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Date:	Sat, 01 May 2010 13:42:59 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	hadi@...erus.ca
Cc:	Changli Gao <xiaosuo@...il.com>,
	David Miller <davem@...emloft.net>, therbert@...gle.com,
	shemminger@...tta.com, netdev@...r.kernel.org,
	Eilon Greenstein <eilong@...adcom.com>,
	Brian Bloniarz <bmb@...enacr.com>
Subject: Re: [PATCH net-next-2.6] net: speedup udp receive path

Le samedi 01 mai 2010 à 07:23 -0400, jamal a écrit :
> On Sat, 2010-05-01 at 07:57 +0200, Eric Dumazet wrote:
> 
> > I changed your program a bit to use EV_PERSIST, (to avoid epoll_ctl()
> > overhead for each packet...)
> 
> Thats a different test case then ;-> You can also get rid of the timer
> (I doubt it will show much difference in results) - I have it in there
> because it i am trying to replicate what i saw causing the regression.
> 
> > RPS off : 220.000 pps 
> > 
> > RPS on (ee mask) : 700.000 pps  (with a slightly modified tg3 driver)
> > 96% of delivered packets
> > 
> 
> That's a very very huge gap. What were the numbers before you changed to
> EV_PERSIST?

But, whole point of epoll is to not change interest each time you get an
event.

Without EV_PERSIST, you need two more syscalls per recvfrom()

epoll_wait()
 epoll_ctl(REMOVE)
 epoll_ctl(ADD)
 recvfrom()

Even poll() would be faster in your case

poll(one fd)
recvfrom()



> Note: i did not add any of your other patches for dst refcnt, sockets
> etc. Were you running with those patches in these tests? I will try the
> next opportunity i get to have latest kernel + those patches. 
> 
> > This is on tg3 adapter, and tg3 has copybreak feature : small packets
> > are copied into skb of the right size.
> 
> Ok, so the driver tuning is also important then (and it shows in the
> profile).

I always thought copybreak was borderline...

It can help to reduce memory footprint (allocating 128 bytes instead of
2048/4096 bytes per frame), but with RPS, it would make sense to perform
copybreak after RPS, not before.

Reducing memory footprint also means less changes on
udp_memory_allocated /tcp_memory_allocate (memory reclaim logic)



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