On Wed, 2016-02-24 at 23:36 -0800, Hugh Dickins wrote: > > 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. 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_*? -- All rights reversed
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