On Wed, 2 Apr 2014, Mel Gorman wrote: > On Thu, Mar 27, 2014 at 06:30:34AM +0800, Shaohua Li wrote: > > > > I posted this patch a year ago or so, but it gets lost. Repost it here to check > > if we can make progress this time. > > > > We use access bit to age a page at page reclaim. When clearing pte access bit, > > we could skip tlb flush in X86. The side effect is if the pte is in tlb and pte > > access bit is unset in page table, when cpu access the page again, cpu will not > > set page table pte's access bit. Next time page reclaim will think this hot > > page is old and reclaim it wrongly, but this doesn't corrupt data. > > > > And according to intel manual, tlb has less than 1k entries, which covers < 4M > > memory. In today's system, several giga byte memory is normal. After page > > reclaim clears pte access bit and before cpu access the page again, it's quite > > unlikely this page's pte is still in TLB. And context swich will flush tlb too. > > The chance skiping tlb flush to impact page reclaim should be very rare. > > > > Originally (in 2.5 kernel maybe), we didn't do tlb flush after clear access bit. > > Hugh added it to fix some ARM and sparc issues. Since I only change this for > > x86, there should be no risk. > > > > And in some workloads, TLB flush overhead is very heavy. In my simple > > multithread app with a lot of swap to several pcie SSD, removing the tlb flush > > gives about 20% ~ 30% swapout speedup. > > > > Signed-off-by: Shaohua Li <shli@xxxxxxxxxxxx> > > I'm aware of the discussion on the more complex version and the outcome > of that. While I think the corner case is real, I think it's also very > unlikely and as this is an x86-only thing which will be safe from > corruption at least; > > Acked-by: Mel Gorman <mgorman@xxxxxxx> > > Shaohua, you almost certainly should resend this to Andrew with the > ack's you collected so that he does not have to dig into the history > trying to figure out what the exact story is. And you can add my Acked-by: Hugh Dickins <hughd@xxxxxxxxxx> to your collection too: you and I discussed this at LSF/MM, and nowadays I agree that the corner case that originally worried me (highly-accessed page not getting its accessed bit updated and then temporarily unmapped) is too unlikely a case to refuse the optimization: it might happen occasionally, but I doubt anybody will notice. Hugh -- 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>