From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756734Ab0IGNek (ORCPT ); Tue, 7 Sep 2010 09:34:40 -0400 Received: from bar.sig21.net ([80.81.252.164]:60787 "EHLO bar.sig21.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754132Ab0IGNee (ORCPT ); Tue, 7 Sep 2010 09:34:34 -0400 Date: Tue, 7 Sep 2010 15:34:29 +0200 From: Johannes Stezenbach To: linux-fsdevel@vger.kernel.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: block cache replacement strategy? Message-ID: <20100907133429.GB3430@sig21.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.20 (2009-06-14) X-Spam-21-Score: -3.8 (---) X-Spam-21-Report: No, score=-3.8 required=5.0 tests=ALL_TRUSTED=-1.8,AWL=0.610,BAYES_00=-2.599 autolearn=no Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, during some simple disk read throughput testing I observed caching behaviour that doesn't seem right. The machine has 2G of RAM and AMD Athlon 4850e, x86_64 kernel but 32bit userspace, Linux 2.6.35.4. It seems that contents of the block cache are not evicted to make room for other blocks. (Or something like that, I have no real clue about this.) Since this is a rather artificial test I'm not too worried, but it looks strange to me so I thought I better report it. zzz:~# echo 3 >/proc/sys/vm/drop_caches zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB) copied, 13.9454 s, 75.2 MB/s zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB) copied, 0.92799 s, 1.1 GB/s OK, seems like the blocks are cached. But: zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000 skip=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB) copied, 13.8375 s, 75.8 MB/s zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000 skip=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB) copied, 13.8429 s, 75.7 MB/s Even if I let 15min pass and repeat the dd command several times, I cannot see any caching effects, it stays at ~75 MB/s. zzz:~# cat /proc/meminfo MemTotal: 1793272 kB MemFree: 15216 kB Buffers: 1378820 kB Cached: 20080 kB SwapCached: 0 kB Active: 792720 kB Inactive: 758832 kB Active(anon): 91716 kB Inactive(anon): 64652 kB Active(file): 701004 kB Inactive(file): 694180 kB But then: zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB) copied, 5.23983 s, 200 MB/s zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB) copied, 0.908284 s, 1.2 GB/s zzz:~# cat /proc/meminfo MemTotal: 1793272 kB MemFree: 16168 kB Buffers: 1377308 kB Cached: 20660 kB SwapCached: 0 kB Active: 1140384 kB Inactive: 410236 kB Active(anon): 91716 kB Inactive(anon): 64652 kB Active(file): 1048668 kB Inactive(file): 345584 kB And finally: zzz:~# echo 3 >/proc/sys/vm/drop_caches zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000 skip=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB) copied, 13.948 s, 75.2 MB/s zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000 skip=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB) copied, 0.985031 s, 1.1 GB/s Now these blocks get cached but then the others don't: zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB) copied, 13.9394 s, 75.2 MB/s zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB) copied, 13.9403 s, 75.2 MB/s Best Regards, Johannes