On Tue, Dec 17, 2013 at 08:54:10AM -0800, Andy Lutomirski wrote: > On Tue, Dec 17, 2013 at 8:04 AM, Alex Thorlton <athorlton@xxxxxxx> wrote: > > On Mon, Dec 16, 2013 at 05:43:40PM -0800, Andy Lutomirski wrote: > >> On Mon, Dec 16, 2013 at 9:12 AM, Alex Thorlton <athorlton@xxxxxxx> wrote: > >> >> Please cc Andrea on this. > >> > > >> > I'm going to clean up a few small things for a v2 pretty soon, I'll be > >> > sure to cc Andrea there. > >> > > >> >> > My proposed solution to the problem is to allow users to set a > >> >> > threshold at which THPs will be handed out. The idea here is that, when > >> >> > a user faults in a page in an area where they would usually be handed a > >> >> > THP, we pull 512 pages off the free list, as we would with a regular > >> >> > THP, but we only fault in single pages from that chunk, until the user > >> >> > has faulted in enough pages to pass the threshold we've set. Once they > >> >> > pass the threshold, we do the necessary work to turn our 512 page chunk > >> >> > into a proper THP. As it stands now, if the user tries to fault in > >> >> > pages from different nodes, we completely give up on ever turning a > >> >> > particular chunk into a THP, and just fault in the 4K pages as they're > >> >> > requested. We may want to make this tunable in the future (i.e. allow > >> >> > them to fault in from only 2 different nodes). > >> >> > >> >> OK. But all 512 pages reside on the same node, yes? Whereas with thp > >> >> disabled those 512 pages would have resided closer to the CPUs which > >> >> instantiated them. > >> > > >> > As it stands right now, yes, since we're pulling a 512 page contiguous > >> > chunk off the free list, everything from that chunk will reside on the > >> > same node, but as I (stupidly) forgot to mention in my original e-mail, > >> > one piece I have yet to add is the functionality to put the remaining > >> > unfaulted pages from our chunk *back* on the free list after we give up > >> > on handing out a THP. Once this is in there, things will behave more > >> > like they do when THP is turned completely off, i.e. pages will get > >> > faulted in closer to the CPU that first referenced them once we give up > >> > on handing out the THP. > >> > >> This sounds like it's almost the worst possible behavior wrt avoiding > >> memory fragmentation. If userspace mmaps a very large region and then > >> starts accessing it randomly, it will allocate a bunch of contiguous > >> 512-page regions, claim one page from each, and return the other 511 > >> pages to the free list. Memory is now maximally fragmented from the > >> point of view of future THP allocations. > > > > Maybe I'm missing the point here to some degree, but the way I think > > about this is that if we trigger the behavior to return the pages to the > > free list, we don't *want* future THP allocations in that range of > > memory for the current process anyways. So, having the memory be > > fragmented from the point of view of future THP allocations isn't an > > issue. > > > > Except that you're causing a problem for the whole system because one > process is triggering the "hugepages aren't helpful" heuristic. I do see where you're coming from here. Do you have any good tests that can cause this type of memory fragmentation that I might be able to take a look at, to see how we might combat that issue in this case? It seems like something that could occur anyways, but my patch would create a situation where it could become a problem much more quickly. Also, just a side note, I see this being more of a problem on a smaller system, where swap is enabled. However, on larger systems where swap is turned off, I think that this scenario might be a bit tougher to hit. I understand that we don't want to hurt the average small system in favor of large ones, but that's why we leave it as a tunable and leave it up to the system administrator to decide whether or not this is appropriate to enable. - Alex -- 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>