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.
