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    SubjectRe: [PATCH] mm/vmstat: retrieve more accurate vmstat value
    On Wed, Jan 27, 2016 at 03:13:12PM -0800, David Rientjes wrote:
    > On Thu, 26 Nov 2015, Joonsoo Kim wrote:
    >
    > > I understand design decision, but, it is better to get value as much
    > > as accurate if there is no performance problem. My patch would not
    > > cause much performance degradation because it is just adding one
    > > this_cpu_read().
    > >
    > > Consider about following example. Current implementation returns
    > > interesting output if someone do following things.
    > >
    > > v1 = zone_page_state(XXX);
    > > mod_zone_page_state(XXX, 1);
    > > v2 = zone_page_state(XXX);
    > >
    > > v2 would be same with v1 in most of cases even if we already update
    > > it.
    > >
    > > This situation could occurs in page allocation path and others. If
    > > some task try to allocate many pages, then watermark check returns
    > > same values until updating vmstat even if some freepage are allocated.
    > > There are some adjustments for this imprecision but why not do it become
    > > accurate? I think that this change is reasonable trade-off.
    > >
    >
    > I'm not sure that NR_ISOLATED_* should be vmstats in the first place. The
    > most important callers that depend on its accuracy is
    > zone_reclaimable_pages() and the too_many_isolated() loop in both
    > shrink_inactive_list() and memory compaction. If zlc's are updated every
    > 1s, the HZ/10 in those loops don't really matter, they may as well be
    > HZ/2.
    >
    > I think memory compaction updates the counters in the most appropriate
    > way, by incrementing a counter and then finally doing
    > mod_zone_page_state() for the counter. The other updaters are thp
    > collapse and page migration.
    >
    > I discount user-visible vmstats here because the trade-off has already
    > been made that they may be stale for up to 1s and userspace isn't
    > affected.
    >
    > So what happens if we simply convert NR_ISOLATED_* into per-zone
    > atomic64_t?

    Just a small uncomfortable thing is that calculation is done
    with different kinds of metric. For example, comparing vmstat values
    (NR_INACTIVE_*, NR_ACTIVE_*) with per-zone atomic NR_ISOLATED_*
    looks ugly and error-prone because their accuracy is different.

    Thanks.

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