On Tue, Jan 31, 2012 at 10:36:53PM -0500, Vivek Goyal wrote: > On Tue, Jan 31, 2012 at 05:22:17PM -0500, Vivek Goyal wrote: > [..] > > > > > > > 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]. > > Ok, I tried it with 2.6.38 kernel and results look more or less same. > Throughput varied between 105MB to 145MB. Many a times it was close to > 110MB and other times it was 145MB. Don't know what causes that spike > sometimes. The block device really has some aged performance bug. Which interestingly only show up in some test environments... > I still see that IO is being submitted one page at a time. The only > real difference seems to be that queue unplug happening at random times > and many a times we are submitting much smaller requests (40 sectors, 48 > sectors etc). Would you share the blktrace data? Thanks, Fengguang -- 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>