netdev - Re: [PATCH] INET : removes per bucket rwlock in tcp/dccp ehash table

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Date:	Thu, 01 Nov 2007 14:46:39 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	dada1@...mosbay.com
Cc:	shemminger@...ux-foundation.org, netdev@...r.kernel.org,
	ak@...e.de, acme@...hat.com
Subject: Re: [PATCH] INET : removes per bucket rwlock in tcp/dccp ehash
 table

From: Eric Dumazet <dada1@...mosbay.com>
Date: Thu, 01 Nov 2007 18:54:24 +0100

> Stephen Hemminger a écrit :
> > Longterm is there any chance of using rcu for this? Seems like
> > it could be a big win.
> 
> This was discussed in the past, and I even believe some patch was proposed, 
> but some guys (including David) complained that RCU is well suited for 'mostly 
>   read' structures.
> 
> On some web server workloads, TCP hash table is constantly accessed in write 
> mode (socket creation, socket move to timewait state, socket  deleted...), and 
> RCU added overhead and poor cache re-use (because sockets must be placed on 
> RCU queue before reuse)
> 
> On these typical workload, hash table without RCU is still the best.

Right, and none of the submitted RCU attempts were correct, it's very
hard to get the synchronization right.

> Longterm changes would rather be based on Robert Olsson suggestion
> last year (trie based lookups and unified IP/TCP cache)
>
> Short term changes would be to be able to resize the TCP hash table (being 
> small at boot, and be able to grow it if necessary). Its current size on 
> modern machines is just insane.

Resizing the hash is also, unfortunately, very hard to implement
as well.

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