On Tue, 15 Jan 2013 16:22:46 -0800 "Darrick J. Wong" <darrick.wong@xxxxxxxxxx> wrote: > > > This patchset has been tested on 3.8.0-rc3 on x64 with ext3, ext4, and xfs. > > > What does everyone think about queueing this for 3.9? > > > > This patchset lacks any performance testing results. > > On my setup (various consumer SSDs and spinny disks, none of which support > T10DIF) I see that the maximum write latency with these patches applied is > about half of what it is without the patches. But don't take my word for it; > Andy Lutomirski[1] says that his soft-rt latency-sensitive programs no longer > freak out when he applies the patch set. Afaik, Google and Taobao run custom > kernels with all this turned off, so they should see similar latency > improvements too. > > Obviously, I see no difference on the DIF disk. We're talking 2001 here ;) Try leaping into your retro time machine and run dbench on ext2 on a spinny disk and I expect you'll see significant performance changes. The problem back in 2001 was that we held lock_page() across the duration of page writeback, so if another thread came in and tried to dirty the page, it would block on lock_page() until IO completion. I can't remember whether writeback would also block read(). Maybe it did, in which case the effects of this patchset won't be as dramatic as were the effects of splitting PG_lock into PG_lock and PG_writeback. > > For clarity's sake, please provide a description of which filesystems > > (and under which circumstances) will block behind writeback when > > userspace is attempting to dirty a page. Both before and, particularly, > > after this patchset. IOW, did everything get fixed? > > Heh, this is complicated. > > Before this patchset, all filesystems would block, regardless of whether or not > it was necessary. ext3 would wait, but still generate occasional checksum > errors. The network filesystems were left to do their own thing, so they'd > wait too. > > After this patchset, all the disk filesystems except ext3 and btrfs will wait > only if the hardware requires it. ext3 (if necessary) snapshots pages instead > of blocking, and btrfs provides its own bdi so the mm will never wait. Network > filesystems haven't been touched, so either they provide their own wait code, > or they don't block at all. The blocking behavior is back to what it was > before 3.0 if you don't have a disk requiring stable page writes. > > (I will reconfirm this statement before sending out the next iteration.) > > I will of course add all of this to the cover message. OK, thanks, that sounds reasonable. Do we generate nice kernel messages (at mount or device-probe time) which will permit people to work out which strategy their device/fs is using? -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
