On Tue, Jan 31, 2012 at 02:13:01PM -0800, Andrew Morton wrote: [..] > > For me, this patch helps only so much and does not get back all the > > performance lost in case of raw disk read. It does improve the throughput > > from around 85-90 MB/s to 110-120 MB/s but running the same dd with > > iflag=direct, gets me more than 250MB/s. > > > > # echo 3 > /proc/sys/vm/drop_caches > > # dd if=/dev/sdb of=/dev/null bs=1M count=1K > > 1024+0 records in > > 1024+0 records out > > 1073741824 bytes (1.1 GB) copied, 9.03305 s, 119 MB/s > > > > echo 3 > /proc/sys/vm/drop_caches > > # dd if=/dev/sdb of=/dev/null bs=1M count=1K iflag=direct > > 1024+0 records in > > 1024+0 records out > > 1073741824 bytes (1.1 GB) copied, 4.07426 s, 264 MB/s > > Buffered I/O against the block device has a tradition of doing Weird > Things. Do you see the same behavior when reading from a regular file? No. Reading file on ext4 file system is working just fine. > > > I think it is happening because in case of raw read we are submitting > > one page at a time to request queue > > (That's not a raw read - it's using pagecache. Please get the terms right!) Ok. > > We've never really bothered making the /dev/sda[X] I/O very efficient > for large I/O's under the (probably wrong) assumption that it isn't a > very interesting case. Regular files will (or should) use the mpage > functions, via address_space_operations.readpages(). fs/blockdev.c > doesn't even implement it. > > > and by the time all the pages > > are submitted and one big merged request is formed it wates lot of time. > > But that was the case in eariler kernels too. Why did it change? Actually, I assumed that the case of reading /dev/sda[X] worked well in earlier kernels. Sorry about that. Will build a 2.6.38 kernel tonight and run the test case again to make sure we had same overhead and relatively poor performance while reading /dev/sda[X]. I think I got confused with Eric's result in another mail where he was reading /dev/sda and getting around 265MB/s with plug removed. And I was wondering that why am I not getting same results. # echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sdb of=/dev/null bs=2M # count=2048 2048+0 enregistrements lus 2048+0 enregistrements écrits 4294967296 octets (4,3 GB) copiés, 16,2309 s, 265 MB/s Maybe something to do with SSD. I will test it anyway with older kernel. Thanks Vivek -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>