Re: [PATCH 06/14] mm, migrate: Immediately fail migration of a page with no migration handler

[Mel Gorman]

On Tue, Dec 18, 2018 at 10:06:31AM +0100, Vlastimil Babka wrote:
> On 12/15/18 12:03 AM, Mel Gorman wrote:
> > Pages with no migration handler use a fallback hander which sometimes
> > works and sometimes persistently fails such as blockdev pages. Migration
> > will retry a number of times on these persistent pages which is wasteful
> > during compaction. This patch will fail migration immediately unless the
> > caller is in MIGRATE_SYNC mode which indicates the caller is willing to
> > wait while being persistent.
> 
> Right.
> 
> > This is not expected to help THP allocation success rates but it does
> > reduce latencies slightly.
> > 
> > 1-socket thpfioscale
> >                                     4.20.0-rc6             4.20.0-rc6
> >                                noreserved-v1r4          failfast-v1r4
> > Amean     fault-both-1         0.00 (   0.00%)        0.00 *   0.00%*
> > Amean     fault-both-3      2276.15 (   0.00%)     3867.54 * -69.92%*
> 
> This is rather weird.
> 

Fault latency is extremely variable and there can be very large outliers
that skew the mean (the full report includes quartiles but it makes for an
excessive changelog). It can be down to luck about how often the migrate
scanner advances and how often it gets reset. For this series, it'll
not be unusual to see jitter in the latencies for individual patches
that will not get nailed down reliably until later in the series. The
alternative is massive patches that do multiple things which will look
nice in changelogs and be horrible to review.

> > Amean     fault-both-5      4992.20 (   0.00%)     5313.20 (  -6.43%)
> > Amean     fault-both-7      7373.30 (   0.00%)     7039.11 (   4.53%)
> > Amean     fault-both-12    11911.52 (   0.00%)    11328.29 (   4.90%)
> > Amean     fault-both-18    17209.42 (   0.00%)    16455.34 (   4.38%)
> > Amean     fault-both-24    20943.71 (   0.00%)    20448.94 (   2.36%)
> > Amean     fault-both-30    22703.00 (   0.00%)    21655.07 (   4.62%)
> > Amean     fault-both-32    22461.41 (   0.00%)    21415.35 (   4.66%)
> > 
> > The 2-socket results are not materially different. Scan rates are
> > similar as expected.
> > 
> > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> 
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> 

Thanks.

-- 
Mel Gorman
SUSE Labs