netdev - Re: Performance regression on kernels 3.10 and newer

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Message-ID: <1408045831.6804.34.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 14 Aug 2014 12:50:31 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: Performance regression on kernels 3.10 and newer

On Thu, 2014-08-14 at 11:46 -0700, Eric Dumazet wrote:

> I believe you answered your own question : prequeue mode does not work
> very well when one host has hundred of active TCP flows to one other.
> 
> In real life, applications do not use prequeue, because nobody wants one
> thread per flow.
> 
> Each socket has its own dst now route cache was removed, but if your
> netperf migrates cpu (and NUMA node), we do not detect the dst should be
> re-created onto a different NUMA node.
> 
> But really, I am not sure we want to care about prequeue, as modern
> applications uses epoll()/poll()/select() instead of blocking on
> recvmsg()

BTW it looks like you do not use RPS/RFS/RSS, so you have different cpus
doing the dst refcount increment and decrement.

With RFS, your workload just works fine.


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