linux-kernel - Re: [PATCH 2.6.23-rc6 Resending] NETWORKING : Edge Triggered EPOLLOUT events get missed for TCP sockets

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Date:	Wed, 19 Sep 2007 16:32:07 -0700 (PDT)
From:	Nagendra Tomar <tomer_iisc@...oo.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	davidel@...ilserver.org
Subject: Re: [PATCH 2.6.23-rc6 Resending] NETWORKING : Edge Triggered EPOLLOUT events get missed for TCP sockets

--- David Miller <davem@...emloft.net> wrote:

> From: Nagendra Tomar <tomer_iisc@...oo.com>
> Date: Wed, 19 Sep 2007 15:55:58 -0700 (PDT)
> 
> >      I agree that setting SOCK_NOSPACE would have been a more elegant
> > fix. Infact I thought a lot about that before deciding on this fix.
> 
> I guess this means you also noticed that you are removing
> the one and only test of this bit too?
> 
> You can't remove this, it's critical for performance.

I'm sure you would have seen value in the check that's why the 
check is there. 

Now we have two critical points to discuss

1. How can we achieve the ET EPOLLOUT event with the SOCK_NOSPACE
   check in place ?
2. How much effect will removing the check have (if we cannot 
   find a way to get the ET EPOLLOUT notification w/ the check
   in place) ? 

Regding (2), IMHO for a "fast sender" the SOCK_NOSPACE check will
almost always pass as the sender will come back to write (or poll)
before the prev data is drained out. If he doesn't do that, he is
not a "fast sender" by definition". A "fast sender" should always
have some data to send when he practically (per the sndbuf space) 
can.

For a "slow sender", do we really care abt the optimization ?

Thanx,
Tomar

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