From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754328Ab0IIMAy (ORCPT ); Thu, 9 Sep 2010 08:00:54 -0400 Received: from bar.sig21.net ([80.81.252.164]:47176 "EHLO bar.sig21.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754073Ab0IIMAx (ORCPT ); Thu, 9 Sep 2010 08:00:53 -0400 Date: Thu, 9 Sep 2010 14:00:44 +0200 From: Johannes Stezenbach To: linux-fsdevel@vger.kernel.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: block cache replacement strategy? Message-ID: <20100909120044.GA27765@sig21.net> References: <20100907133429.GB3430@sig21.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100907133429.GB3430@sig21.net> 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=0.602,BAYES_00=-2.599 autolearn=no Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 07, 2010 at 03:34:29PM +0200, Johannes Stezenbach wrote: > > 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. C'mon guys, please comment. Is this a bug or not? Or is my question too silly? Johannes > 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