Re: [PATCH v2 0/9] workingset protection/detection on the anonymous LRU list

[Aaron Lu]

On Thu, Feb 27, 2020 at 08:48:06AM -0500, Johannes Weiner wrote:
> On Wed, Feb 26, 2020 at 07:39:42PM -0800, Andrew Morton wrote:
> > It sounds like the above simple aging changes provide most of the
> > improvement, and that the workingset changes are less beneficial and a
> > bit more risky/speculative?
> > 
> > If so, would it be best for us to concentrate on the aging changes
> > first, let that settle in and spread out and then turn attention to the
> > workingset changes?
> 
> Those two patches work well for some workloads (like the benchmark),
> but not for others. The full patchset makes sure both types work well.
> 
> Specifically, the existing aging strategy for anon assumes that most
> anon pages allocated are hot. That's why they all start active and we
> then do second-chance with the small inactive LRU to filter out the
> few cold ones to swap out. This is true for many common workloads.
> 
> The benchmark creates a larger-than-memory set of anon pages with a
> flat access profile - to the VM a flood of one-off pages. Joonsoo's

test: swap-w-rand-mt, which is a multi thread swap write intensive
workload so there will be swap out and swap ins.

> first two patches allow the VM to usher those pages in and out of

Weird part is, the robot says the performance gain comes from the 1st
patch only, which adjust the ratio, not including the 2nd patch which
makes anon page starting from inactive list.

I find the performance gain hard to explain...

> memory very quickly, which explains the throughput boost. But it comes
> at the cost of reducing space available to hot anon pages, which will
> regress others.
>