On 1/17/19 8:39 PM, Mel Gorman wrote: > On Thu, Jan 17, 2019 at 06:58:30PM +0100, Vlastimil Babka wrote: >> On 1/4/19 1:50 PM, Mel Gorman wrote: >> > The fast isolation of pages can move the scanner faster than is necessary >> > depending on the contents of the free list. This patch will only allow >> > the fast isolation to initialise the scanner and advance it slowly. The >> > primary means of moving the scanner forward is via the linear scanner >> > to reduce the likelihood the migration source/target scanners meet >> > prematurely triggering a rescan. >> >> Maybe I've seen enough code today and need to stop, but AFAICS the description >> here doesn't match the actual code changes? What I see are some cleanups, and a >> change in free scanner that will set pageblock skip bit after a pageblock has >> been scanned, even if there were pages isolated, while previously it would set >> the skip bit only if nothing was isolated. >> > > The first three hunks could have been split out but it wouldn't help > overall. Maybe a changelog rewrite will help; > > mm, compaction: Reduce premature advancement of the migration target scanner > > The fast isolation of free pages allows the cached PFN of the free > scanner to advance faster than necessary depending on the contents > of the free list. The key is that fast_isolate_freepages() can update > zone->compact_cached_free_pfn via isolate_freepages_block(). When the > fast search fails, the linear scan can start from a point that has skipped > valid migration targets, particularly pageblocks with just low-order > free pages. This can cause the migration source/target scanners to meet > prematurely causing a reset. > > This patch starts by avoiding an update of the pageblock skip information > and cached PFN from isolate_freepages_block() and puts the responsibility > of updating that information in the callers. The fast scanner will update > the cached PFN if and only if it finds a block that is higher than the > existing cached PFN and sets the skip if the pageblock is full or nearly > full. The linear scanner will update skipped information and the cached > PFN only when a block is completely scanned. The total impact is that > the free scanner advances more slowly as it is primarily driven by the > linear scanner instead of the fast search. > > Does that help? Yeah, now I get it, thanks! Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
