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. -- 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>