On Tue, Jan 15, 2013 at 04:33:59PM -0800, Andrew Morton wrote: > 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. Now that you've stirred my memory, I /do/ dimly recall that Linux waited for writeback back in the old days. At least we'll be back to that. As a side note, the average latency of a write to a non-DIF disk dropped down to nearly nothing. > > > 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? No. /sys/devices/virtual/bdi/*/stable_pages_required will tell you stable pages are on or not, but so far only ext3 uses snapshots and the rest just wait. Do you think a printk would be useful? --D -- 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
