netdev - Re: Choppy TCP send performance

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Date:	Sat, 29 May 2010 00:00:55 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ivan Novick <novickivan@...il.com>
Cc:	netdev@...r.kernel.org, Tim Heath <theath@...enplum.com>
Subject: Re: Choppy TCP send performance

Le vendredi 28 mai 2010 à 14:35 -0700, Ivan Novick a écrit :
> On Fri, May 28, 2010 at 2:16 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > Le vendredi 28 mai 2010 à 13:38 -0700, Ivan Novick a écrit :
> >> sk_stream_wait_memory seems to be called when the send buffer is full
> >> and the next send call does not complete until the send buffer
> >> utilization goes down from 4,194,304 bytes to 2,814,968 bytes.
> >>
> >> This implies that the send that blocks on a full send buffer will not
> >> complete until there is 1 meg of free space in the send buffer even
> >> though the send could be accepted into the OS with only 128KB of free
> >> space.
> >>
> >
> > static void sock_def_write_space(struct sock *sk)
> > {
> > ...
> > if ((atomic_read(&sk->sk_wmem_alloc) << 1) <= sk->sk_sndbuf) {
> > ...
> >
> >
> > Quick answer is : No, this is not tunable ( independantly than SNDBUF )
> >
> > SO_SNDLOWAT is not implemented on linux, yet (its value is : 1).
> >
> >
> > Why would you want to wakeup your thread more than necessary ?
> 
> Cool.  This helps me understand what is happening.
> 
> My user thread wants to wake up as soon as the OS can accept my data
> so that it can continue doing work and interact with other components
> in the system.  This is an application issue, i can work around it now
> that i have a better understanding of what the kernel is doing.

If you use poll() or select() before issuing your write(), I believe it
should be OK.



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