From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Subject: Re: cat /proc/net/tcp takes 0.5 seconds on x86_64 Date: Wed, 27 Aug 2008 23:51:58 -0700 (PDT) Message-ID: <20080827.235158.187658055.davem@davemloft.net> References: <20080827144800.5f9fc5b4@extreme> <20080827.150955.118944272.davem@davemloft.net> <48B643C3.9040502@cosmosbay.com> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: shemminger@vyatta.com, andi@firstfloor.org, davej@redhat.com, netdev@vger.kernel.org, j.w.r.degoede@hhs.nl To: dada1@cosmosbay.com Return-path: Received: from 74-93-104-97-Washington.hfc.comcastbusiness.net ([74.93.104.97]:33551 "EHLO sunset.davemloft.net" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1752466AbYH1GwD (ORCPT ); Thu, 28 Aug 2008 02:52:03 -0400 In-Reply-To: <48B643C3.9040502@cosmosbay.com> Sender: netdev-owner@vger.kernel.org List-ID: From: Eric Dumazet Date: Thu, 28 Aug 2008 08:20:51 +0200 > But for route cache, it is probably doable since we added the > rt_genid thing in commit 29e75252da20f3ab9e132c68c9aed156b87beae6 > ([IPV4] route cache: Introduce rt_genid for smooth cache > invalidation) > > If we add a hash table for each "struct net" > (net->ipv4.rt_hash_table), we then could do something sensible when > an admin writes to /proc/sys/net/ipv4/route/hash_size or at > rt_check_expire() time, if hash table is found to be full... The synchronization and implementation is not a problem for the route cache, I implemented this eons ago. > 3) In rt_check_expire(), adds some metrics to trigger an expand of the > hash table in case we found too many entries in it. This is the problem and why I didn't just commit the patch I had back then. We could not define a reasonable way to trigger hash table growth. GC attempts to keep a resident set of entries in the cache, and these heuristics are guided by the table size itself. So if you grow the table too aggressively this never has a chance to work. You want to respond dynamically to traffic in a reasonable amount of time, but you don't want to get tricked by bursts of RCU effects. We never came up with an algorithm that addresses all of these issues.