On Tue, 2 May 2017 13:52:00 -0300
Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> > I have several questions about the tunables:
> >
> > - What does the vmstat_threshold value mean? What are the implications
> > of changing this value? What's the difference in choosing 1, 2, 3
> > or 500?
>
> Its the maximum value for a vmstat statistics counter to hold. After
> that value, the statistics are transferred to the global counter:
>
> void __mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
> long delta)
> {
> struct per_cpu_nodestat __percpu *pcp = pgdat->per_cpu_nodestats;
> s8 __percpu *p = pcp->vm_node_stat_diff + item;
> long x;
> long t;
>
> x = delta + __this_cpu_read(*p);
>
> t = __this_cpu_read(pcp->stat_threshold);
>
> if (unlikely(x > t || x < -t)) {
> node_page_state_add(x, pgdat, item);
> x = 0;
> }
> __this_cpu_write(*p, x);
> }
> EXPORT_SYMBOL(__mod_node_page_state);
>
> BTW, there is a bug there, should change that to:
>
> if (unlikely(x >= t || x <= -t)) {
>
> Increasing the threshold value does two things:
> 1) It decreases the number of inter-processor accesses.
> 2) It increases how much the global counters stay out of
> sync relative to actual current values.
OK, but I'm mostly concerned with the sysadmin who will have
to change the tunable. So, I think it's a good idea to improve
the doc to contain that information.
> > - If the purpose of having vmstat_threshold is to allow disabling
> > the vmstat kworker, why can't the kernel pick a value automatically?
>
> Because it might be acceptable for the user to accept a small
> out of syncedness of the global counters in favour of performance
> (one would have to analyze the situation).
>
> Setting vmstat_threshold == 1 means the global counter is always
> in sync with the page counter state of the pCPU.
IMHO, if vmstat_threshold == 1 is the required setting for
disabling the vmstat kworker then I'd go with only one tunable
for now. But that's just a suggestion.
>
> > - What are the implications of disabling the vmstat kworker? Will vm
> > stats still be collected someway or will it be completely off for
> > the CPU?
>
> It will not be necessary to collect vmstats because at every modification
> of the vm statistics, pCPUs with vmstat_threshold=1 transfer their
> values to the global counters (that is, there is no queueing of statistics
> locally to improve performance).
Ah, OK. Got this now. I'll give this patch a try. But I think we want
to hear from Christoph (who worked on reducing the vmstat interruptions
in the past).
> > Also, shouldn't this patch be split into two?
>
> First add one sysfs file, then add another sysfs file, you mean?
Yes, one tunable per patch.
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