On Mon, 6 Jul 2015 14:39:56 +0100 Mel Gorman <mgorman@xxxxxxx> wrote: > Pages that are unmapped for reclaim must be flushed before being freed to > avoid corruption due to a page being freed and reallocated while a stale > TLB entry exists. When reclaiming mapped pages, the requires one IPI per > SWAP_CLUSTER_MAX. This patch increases SWAP_CLUSTER_MAX to 256 so more > pages can be flushed with a single IPI. This number was selected because > it reduced IPIs for TLB shootdowns by 40% on a workload that is dominated > by mapped pages. > > Note that it is expected that doubling SWAP_CLUSTER_MAX would not always > halve the IPIs as it is workload dependent. Reclaim efficiency was not 100% > on this workload which was picked for being IPI-intensive and was closer to > 35%. More importantly, reclaim does not always isolate in SWAP_CLUSTER_MAX > pages. The LRU lists for a zone may be small, the priority can be low > and even when reclaiming a lot of pages, the last isolation may not be > exactly SWAP_CLUSTER_MAX. > > There are a few potential issues with increasing SWAP_CLUSTER_MAX. > > 1. LRU lock hold times increase slightly because more pages are being > isolated. > 2. There are slight timing changes due to more pages having to be > processed before they are freed. There is a slight risk that more > pages than are necessary get reclaimed. > 3. There is a risk that too_many_isolated checks will be easier to > trigger resulting in a HZ/10 stall. > 4. The rotation rate of active->inactive is slightly faster but there > should be fewer rotations before the lists get balanced so it > shouldn't matter. > 5. More pages are reclaimed in a single pass if zone_reclaim_mode is > active but that thing sucks hard when it's enabled no matter what > 6. More pages are isolated for compaction so page hold times there > are longer while they are being copied > > It's unlikely any of these will be problems but worth keeping in mind if > there are any reclaim-related bug reports in the near future. Yes, this may well cause small&subtle changes which will take some time to be noticed. What is the overall effect on the performance improvement if this patch is omitted? I wonder if we should leave small systems or !SMP systems or CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH=n systems with SWAP_CLUSTER_MAX=32. If not, why didn't we change this years ago ;) -- 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>