linux-kernel - Re: [PATCH] MM : alloc_large_system_hash() can free some memory for non power-of-two bucketsize

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Date:	Sat, 19 May 2007 20:41:01 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	William Lee Irwin III <wli@...omorphy.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	linux kernel <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH] MM : alloc_large_system_hash() can free some memory for
 non power-of-two bucketsize

William Lee Irwin III a écrit :
> On Fri, May 18, 2007 at 11:54:54AM +0200, Eric Dumazet wrote:
>> alloc_large_system_hash() is called at boot time to allocate space
>> for several large hash tables.
>> Lately, TCP hash table was changed and its bucketsize is not a
>> power-of-two anymore.
>> On most setups, alloc_large_system_hash() allocates one big page
>> (order > 0) with __get_free_pages(GFP_ATOMIC, order). This single
>> high_order page has a power-of-two size, bigger than the needed size.
>> We can free all pages that wont be used by the hash table.
>> On a 1GB i386 machine, this patch saves 128 KB of LOWMEM memory.
>> TCP established hash table entries: 32768 (order: 6, 393216 bytes)
> 
> The proper way to do this is to convert the large system hashtable
> users to use some data structure / algorithm  other than hashing by
> separate chaining.

No thanks. This was already discussed to death on netdev. To date, hash tables 
are a good compromise.

I dont mind losing part of memory, I prefer to keep good performance when 
handling 1.000.000 or more tcp sessions.

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