Re: [RFC v5 0/3] mm: make swapin readahead to gain more thp performance

in Thu, Feb 25, 2016 at 05:35:50PM -0500, Rik van Riel wrote:

On Wed, 2016-02-24 at 23:36 -0800, Hugh Dickins wrote:

quoted

Doesn't this imply that __collapse_huge_page_swapin() will initiate
all
the necessary swapins for a THP, then (given the
FAULT_FLAG_ALLOW_RETRY)
not wait for them to complete, so khugepaged will give up on that
extent
and move on to another; then after another full circuit of all the
mms
it needs to examine, it will arrive back at this extent and build a
THP
from the swapins it arranged last time.

Which may work well when a system transitions from busy+swappingout
to idle+swappingin, but isn't that rather a special case?  It feels
(meaning, I've not measured at all) as if the inbetween busyish case
will waste a lot of I/O and memory on swapins that have to be
discarded
again before khugepaged has made its sedate way back to slotting them
in.

There may be a fairly simple way to prevent
that from becoming an issue.

When khugepaged wakes up, it can check the
PGSWPOUT or even the PGSTEAL_* stats for
the system, and skip swapin readahead if
there was swapout activity (or any page
reclaim activity?) since the time it last
ran.

That way the swapin readahead will do
its thing when transitioning from
busy + swapout to idle + swapin, but not
while the system is under permanent memory
pressure.

The idea make sense for me.

Am I forgetting anything obvious?

Is this too aggressive?

Not aggressive enough?

Could PGPGOUT + PGSWPOUT be a useful
in-between between just PGSWPOUT or
PGSTEAL_*?

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