Received: from magus.postgresql.org (magus.postgresql.org [87.238.57.229]) by mail.postgresql.org (Postfix) with ESMTP id B9D274BB8EB for ; Wed, 4 Apr 2012 13:54:21 -0300 (ADT) Received: from [2001:1528:137::f1] (helo=mail.gransy.com) by magus.postgresql.org with esmtp (Exim 4.72) (envelope-from ) id 1SFTTO-0004yq-Tm for pgsql-performance@postgresql.org; Wed, 04 Apr 2012 16:54:20 +0000 Received: from antispam.gransy.com (antispam.gransy.com [77.78.104.124]) by mail.gransy.com (Postfix) with ESMTP id 30A035088A0; Wed, 4 Apr 2012 18:54:06 +0200 (CEST) Received: from mail.gransy.com ([82.208.29.194]) by antispam.gransy.com (antispam.gransy.com [77.78.104.124]) (amavisd-new, port 10024) with ESMTP id LoXKdyUNsixe; Wed, 4 Apr 2012 18:54:06 +0200 (CEST) Received: by mail.gransy.com (Postfix, from userid 5000) id 14A575088B0; Wed, 4 Apr 2012 18:54:06 +0200 (CEST) Received: from [192.168.1.237] (ip-78-45-133-255.net.upcbroadband.cz [78.45.133.255]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by mail.gransy.com (Postfix) with ESMTPSA id DDFCE5088A0; Wed, 4 Apr 2012 18:54:05 +0200 (CEST) Message-ID: <4F7C7CAE.40306@fuzzy.cz> Date: Wed, 04 Apr 2012 18:54:06 +0200 From: Tomas Vondra User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120312 Thunderbird/11.0 MIME-Version: 1.0 To: Scott Marlowe CC: pgsql-performance@postgresql.org Subject: Re: H800 + md1200 Performance problem References: <920BB532-C321-45A3-851E-8B688A5963F7@yahoo.com> <4F7AF966.20809@fuzzy.cz> <4F7B3AEC.4000204@fuzzy.cz> <4F7C4AAE.4060608@fuzzy.cz> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Host-Lookup-Failed: Reverse DNS lookup failed for 2001:1528:137::f1 (failed) X-Pg-Spam-Score: -1.1 (-) X-Archive-Number: 201204/47 X-Sequence-Number: 46693 On 4.4.2012 18:22, Scott Marlowe wrote: > On Wed, Apr 4, 2012 at 7:20 AM, Tomas Vondra wrote: >> On 4.4.2012 15:15, Scott Marlowe wrote: >>> On Wed, Apr 4, 2012 at 3:42 AM, Cesar Martin wrote: >>>> >>>> I have noticed that since I changed the setting vm.zone_reclaim_mode = 0, >>>> swap is totally full. Do you recommend me disable swap? >>> >>> Yes >> >> Careful about that - it depends on how you disable it. >> >> Setting 'vm.swappiness = 0' is a good idea, don't remove the swap (I've >> been bitten by the vm.overcommit=2 without a swap repeatedly). > > I've had far more problems with swap on and swappiness set to 0 than > with swap off. But this has always been on large memory machines with > 64 to 256G memory. Even with fairly late model linux kernels (i.e. > 10.04 LTS through 11.04) I've watched the kswapd start up swapping > hard on a machine with zero memory pressure and no need for swap. > Took about 2 weeks of hard running before kswapd decided to act > pathological. > > Seen it with swap on, with swappiness to 0, and overcommit to either 0 > or 2 on big machines. Once we just took the swap partitions away it > the machines ran fine. I've experienced the issues in exactly the opposite case - machines with very little memory (like a VPS with 512MB of RAM). I did want to operate that machine without a swap yet it kept failing because of OOM errors or panicking (depending on the overcommit ratio value). Turns out it's quite difficult (~ almost impossible) tune the VM for a swap-less case. In the end I've just added a 256MB of swap and everything started to work fine - funny thing is the swap is not used at all (according to sar). T.