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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
