Re: [PATCH 0/2] /proc/stat: Reduce irqs counting performance overhead

From: Dave Chinner
Date: Mon Jan 07 2019 - 21:04:29 EST

On Mon, Jan 07, 2019 at 05:41:39PM -0500, Waiman Long wrote:
> On 01/07/2019 05:32 PM, Dave Chinner wrote:
> > On Mon, Jan 07, 2019 at 10:12:56AM -0500, Waiman Long wrote:
> >> As newer systems have more and more IRQs and CPUs available in their
> >> system, the performance of reading /proc/stat frequently is getting
> >> worse and worse.
> > Because the "roll-your-own" per-cpu counter implementaiton has been
> > optimised for low possible addition overhead on the premise that
> > summing the counters is rare and isn't a performance issue. This
> > patchset is a direct indication that this "summing is rare and can
> > be slow" premise is now invalid.
> >
> > We have percpu counter infrastructure that trades off a small amount
> > of addition overhead for zero-cost reading of the counter value.
> > i.e. why not just convert this whole mess to percpu_counters and
> > then just use percpu_counter_read_positive()? Then we just don't
> > care how often userspace reads the /proc file because there is no
> > summing involved at all...
> >
> > Cheers,
> >
> > Dave.
> Yes, percpu_counter_read_positive() is cheap. However, you still need to
> pay the price somewhere. In the case of percpu_counter, the update is
> more expensive.

Ummm, that's exactly what I just said. It's a percpu counter that
solves the "sum is expensive and frequent" problem, just like you
are encountering here. I do not need basic scalability algorithms
explained to me.

> I would say the percentage of applications that will hit this problem is
> small. But for them, this problem has some significant performance overhead.

Well, duh!

What I was suggesting is that you change the per-cpu counter
implementation to the /generic infrastructure/ that solves this
problem, and then determine if the extra update overhead is at all
measurable. If you can't measure any difference in update overhead,
then slapping complexity on the existing counter to attempt to
mitigate the summing overhead is the wrong solution.

Indeed, it may be that you need o use a custom batch scaling curve
for the generic per-cpu coutner infrastructure to mitigate the
update overhead, but the fact is we already have generic
infrastructure that solves your problem and so the solution should
be "use the generic infrastructure" until it can be proven not to

i.e. prove the generic infrastructure is not fit for purpose and
cannot be improved sufficiently to work for this use case before
implementing a complex, one-off snowflake counter implementation...


Dave Chinner