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vm_pageout: Avoid rounding down the inactive scan target

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With helper page daemon threads, enabled by default in r364786, we
divide the inactive target by the number of threads, rounding down, and
sum the total number of pages freed by the threads.  This sum is
compared with the original target, but by rounding down we might lose
pages, causing the page daemon control loop to conclude that inactive
queue scanning isn't keeping up with demand for free pages.  Typically
this results in excessive swapping.

Fix the problem by accounting for the error in the main pagedaemon
thread's target.  Note that by default the problem will manifest only in
systems with >16 CPUs in a NUMA domain.

Reviewed by:	cem
Discussed with:	dougm
Reported and tested by:	dhw, glebius
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D26610
This commit is contained in:
Mark Johnston 2020-10-02 19:16:06 +00:00
parent 06d8bdcbf7
commit 2913cc4637
1 changed files with 8 additions and 7 deletions
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15
sys/vm/vm_pageout.c
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@ -1649,25 +1649,26 @@ vm_pageout_scan_inactive(struct vm_domain *vmd, int page_shortage)
/*
* Dispatch a number of inactive threads according to load and collect the
* results to prevent a coherent (CEM: incoherent?) view of paging activity on
* this domain.
* results to present a coherent view of paging activity on this domain.
*/
static int
vm_pageout_inactive_dispatch(struct vm_domain *vmd, int shortage)
{
u_int freed, pps, threads, us;
u_int freed, pps, slop, threads, us;
vmd->vmd_inactive_shortage = shortage;
slop = 0;
/*
* If we have more work than we can do in a quarter of our interval, we
* fire off multiple threads to process it.
*/
if (vmd->vmd_inactive_threads > 1 && vmd->vmd_inactive_pps != 0 &&
threads = vmd->vmd_inactive_threads;
if (threads > 1 && vmd->vmd_inactive_pps != 0 &&
shortage > vmd->vmd_inactive_pps / VM_INACT_SCAN_RATE / 4) {
threads = vmd->vmd_inactive_threads;
vm_domain_pageout_lock(vmd);
vmd->vmd_inactive_shortage /= threads;
slop = shortage % threads;
vm_domain_pageout_lock(vmd);
blockcount_acquire(&vmd->vmd_inactive_starting, threads - 1);
blockcount_acquire(&vmd->vmd_inactive_running, threads - 1);
wakeup(&vmd->vmd_inactive_shortage);
@ -1675,7 +1676,7 @@ vm_pageout_inactive_dispatch(struct vm_domain *vmd, int shortage)
}
/* Run the local thread scan. */
vm_pageout_scan_inactive(vmd, vmd->vmd_inactive_shortage);
vm_pageout_scan_inactive(vmd, vmd->vmd_inactive_shortage + slop);
/*
* Block until helper threads report results and then accumulate