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 -- 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
