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

[Aaron Lu]

On Fri, Feb 28, 2020 at 01:03:03PM +0900, Joonsoo Kim wrote:
> Hello,
> 
> On Fri, Feb 28, 2020 at 11:23:58AM +0800, Aaron Lu wrote:
> > 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...
> 
> Let me explain the reason of the performance gain.
> 
> 1st patch provides more second chance to the anonymous pages.

By second chance, do I understand correctely this refers to pages on 
inactive list get moved back to active list?

> In swap-w-rand-mt test, memory used by all threads is greater than the
> amount of the system memory, but, memory used by each thread would
> not be much. So, although it is a rand test, there is a locality
> in each thread's job. More second chance helps to exploit this
> locality so performance could be improved.

Does this mean there should be fewer vmstat.pswpout and vmstat.pswpin
with patch1 compared to vanilla?

Thanks.