On Wed, 2011-05-11 at 16:29 +0100, Mel Gorman wrote: > Debian (and probably Ubuntu) have recently have changed to the default > option of SLUB. There are a few reports of people experiencing hangs > when copying large amounts of data with kswapd using a large amount of > CPU. It appears this is down to SLUB using high orders by default and > the page allocator and reclaim struggling to keep up. The following > three patches reduce the cost of using those high orders. > > Patch 1 prevents kswapd waking up in response to SLUBs speculative > use of high orders. This eliminates the hangs and while the > system can still stall for long periods, it recovers. > > Patch 2 further reduces the cost by prevent SLUB entering direct > compaction or reclaim paths on the grounds that falling > back to order-0 should be cheaper. > > Patch 3 defaults SLUB to using order-0 on the grounds that the > systems that heavily benefit from using high-order are also > sized to fit in physical memory. On such systems, they should > manually tune slub_max_order=3. > > My own data on this is not great. I haven't really been able to > reproduce the same problem locally but a significant failing is > that the tests weren't stressing X but I couldn't make meaningful > comparisons by just randomly clicking on things (working on fixing > this problem). > > The test case is simple. "download tar" wgets a large tar file and > stores it locally. "unpack" is expanding it (15 times physical RAM > in this case) and "delete source dirs" is the tarfile being deleted > again. I also experimented with having the tar copied numerous times > and into deeper directories to increase the size but the results were > not particularly interesting so I left it as one tar. > > Test server, 4 CPU threads (AMD Phenom), x86_64, 2G of RAM, no X running > - nowake > largecopy-vanilla kswapd-v1r1 noexstep-v1r1 default0-v1r1 > download tar 94 ( 0.00%) 94 ( 0.00%) 94 ( 0.00%) 93 ( 1.08%) > unpack tar 521 ( 0.00%) 551 (-5.44%) 482 ( 8.09%) 488 ( 6.76%) > delete source dirs 208 ( 0.00%) 218 (-4.59%) 194 ( 7.22%) 194 ( 7.22%) > MMTests Statistics: duration > User/Sys Time Running Test (seconds) 740.82 777.73 739.98 747.47 > Total Elapsed Time (seconds) 1046.66 1273.91 962.47 936.17 > > Disabling kswapd alone hurts performance slightly even though testers > report it fixes hangs. I would guess it's because SLUB callers are > calling direct reclaim more frequently (I belatedly noticed that > compaction was disabled so it's not a factor) but haven't confirmed > it. However, preventing kswapd waking or entering direct reclaim and > having SLUB falling back to order-0 performed noticeably faster. Just > using order-0 in the first place was fastest of all. > > I tried running the same test on a test laptop but unfortunately > due to a misconfiguration the results were lost. It would take a few > hours to rerun so am posting without them. > > If the testers verify this series help and we agree the patches are > appropriate, they should be considered a stable candidate for 2.6.38. OK, I confirm that I can't seem to break this one. No hangs visible, even when loading up the system with firefox, evolution, the usual massive untar, X and even a distribution upgrade. You can add my tested-by James -- 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
