From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cantor2.suse.de ([195.135.220.15]:55228 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753375Ab3H2MpL (ORCPT ); Thu, 29 Aug 2013 08:45:11 -0400 Date: Thu, 29 Aug 2013 14:45:10 +0200 From: David Sterba To: Miao Xie Cc: linux-btrfs@vger.kernel.org Subject: Re: [PATCH] Btrfs: allocate the free space by the existed max extent size when ENOSPC Message-ID: <20130829124510.GH23113@twin.jikos.cz> Reply-To: dsterba@suse.cz References: <1377755258-2722-1-git-send-email-miaox@cn.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <1377755258-2722-1-git-send-email-miaox@cn.fujitsu.com> Sender: linux-btrfs-owner@vger.kernel.org List-ID: On Thu, Aug 29, 2013 at 01:47:38PM +0800, Miao Xie wrote: > By the current code, if the requested size is very large, and all the extents > in the free space cache are small, we will waste lots of the cpu time to cut > the requested size in half and search the cache again and again until it gets > down to the size the allocator can return. In fact, we can know the max extent > size in the cache after the first search, so we needn't cut the size in half > repeatedly, and just use the max extent size directly. This way can save > lots of cpu time and make the performance grow up when there are only fragments > in the free space cache. > > According to my test, if there are only 4KB free space extents in the fs, > and the total size of those extents are 256MB, we can reduce the execute > time of the following test from 5.4s to 1.4s. > dd if=/dev/zero of= bs=1MB count=1 oflag=sync Sounds like a good improvement! Can you please post the test? Fragmented free space is nothing uncommon, so I guess aging the filesystem for a while works as well and there the improvement would show up in reduced cpu time. Patch looks ok. david