On 8/13/2013 6:26 PM, Andrew Morton wrote: > On Tue, 13 Aug 2013 18:13:48 -0400 Chris Metcalf <cmetcalf@xxxxxxxxxx> wrote: > >> On 8/13/2013 5:13 PM, Andrew Morton wrote: >>> On Tue, 13 Aug 2013 16:59:54 -0400 Chris Metcalf <cmetcalf@xxxxxxxxxx> wrote: >>> >>>>> Then again, why does this patchset exist? It's a performance >>>>> optimisation so presumably someone cares. But not enough to perform >>>>> actual measurements :( >>>> The patchset exists because of the difference between zero overhead on >>>> cpus that don't have drainable lrus, and non-zero overhead. This turns >>>> out to be important on workloads where nohz cores are handling 10 Gb >>>> traffic in userspace and really, really don't want to be interrupted, >>>> or they drop packets on the floor. >>> But what is the effect of the patchset? Has it been tested against the >>> problematic workload(s)? >> Yes. The result is that syscalls such as mlockall(), which otherwise interrupt >> every core, don't interrupt the cores that are running purely in userspace. >> Since they are purely in userspace they don't have any drainable pagevecs, >> so the patchset means they don't get interrupted and don't drop packets. >> >> I implemented this against Linux 2.6.38 and our home-grown version of nohz >> cpusets back in July 2012, and we have been shipping it to customers since then. > argh. > > Those per-cpu LRU pagevecs were a nasty but very effective locking > amortization hack back in, umm, 2002. They have caused quite a lot of > weird corner-case behaviour, resulting in all the lru_add_drain_all() > calls sprinkled around the place. I'd like to nuke the whole thing, > but that would require a fundamental rethnik/rework of all the LRU list > locking. > > According to the 8891d6da17db0f changelog, the lru_add_drain_all() in > sys_mlock() isn't really required: "it isn't must. but it reduce the > failure of moving to unevictable list. its failure can rescue in > vmscan later. but reducing is better." > > I suspect we could just kill it. That's probably true, but I suspect this change is still worthwhile for nohz environments. There are other calls of lru_add_drain_all(), and you just don't want anything in the kernel that interrupts every core when only a subset could be interrupted. If the kernel can avoid generating unnecessary interrupts to uninvolved cores, you can make guarantees about jitter on cores that are running dedicated userspace code. -- Chris Metcalf, Tilera Corp. http://www.tilera.com -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>