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

From: David Miller <[email-masked]>
Date: Thu, 01 Nov 2007 04:03:40 -0700 (PDT)

From: Eric Dumazet <[email-masked]>
Date: Thu, 01 Nov 2007 11:16:20 +0100

> As done two years ago on IP route cache table (commit 
> 22c047ccbc68fa8f3fa57f0e8f906479a062c426) , we can avoid using one lock per 
> hash bucket for the huge TCP/DCCP hash tables.
> 
> On a typical x86_64 platform, this saves about 2MB or 4MB of ram, for litle 
> performance differences. (we hit a different cache line for the rwlock, but 
> then the bucket cache line have a better sharing factor among cpus, since we 
> dirty it less often)
> 
> Using a 'small' table of hashed rwlocks should be more than enough to provide 
> correct SMP concurrency between different buckets, without using too much 
> memory. Sizing of this table depends on NR_CPUS and various CONFIG settings.
> 
> This patch provides some locking abstraction that may ease a future work using 
>   a different model for TCP/DCCP table.
> 
> Signed-off-by: Eric Dumazet <[email-masked]>

Nice work Eric.

I've tossed this into my local tree and we'll let this cook
for a few days.  If no problems crop up I will submit it
for 2.6.24 because the memory savings is non-trivial.