On Fri, Sep 10, 2010 at 12:02:35PM +0200, Florian Mickler wrote: > > 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. > > Well I personally have no clue about the block caching, but perhaps > that is an heuristic to prevent the cache from fluctuating too much? > Some minimum time a block is hold... in a big linear read the cache is > useless anyway most of the time, so it could make some sense... > > You could try accessing random files after filling up the cache and > check if those evict the the cache. That should rule out any > linear-read-detection heuristic. OK, here is another run with simple files (using two kvm images I had lying around). Note how the cache used by /dev/sda2 apparently prevents the kvm image from being cached, but also how the cache used by test.img prevents test2.img from being cached. Linear read heuristic might be a good guess, but it would be nice to hear a comment from a vm/fs expert which confirms this works as intended. Thanks 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, 13.9516 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, 0.957778 s, 1.1 GB/s zzz:~# dd if=~js/qemu/test.img of=/dev/null bs=1M count=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB) copied, 18.4247 s, 56.9 MB/s zzz:~# dd if=~js/qemu/test.img of=/dev/null bs=1M count=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB) copied, 18.3675 s, 57.1 MB/s zzz:~# dd if=~js/qemu/test.img of=/dev/null bs=1M count=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB) copied, 18.3925 s, 57.0 MB/s zzz:~# echo 3 >/proc/sys/vm/drop_caches zzz:~# dd if=~js/qemu/test.img of=/dev/null bs=1M count=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB) copied, 18.5455 s, 56.5 MB/s zzz:~# dd if=~js/qemu/test.img of=/dev/null bs=1M count=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB) copied, 0.950387 s, 1.1 GB/s zzz:~# dd if=~js/qemu/test2.img of=/dev/null bs=1M count=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB) copied, 18.085 s, 58.0 MB/s zzz:~# dd if=~js/qemu/test2.img of=/dev/null bs=1M count=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB) copied, 17.7351 s, 59.1 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
