Re: [PATCH 20/25] mm, compaction: Reduce unnecessary skipping of migration target scanner

[Vlastimil Babka]

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>