Re: [RFC PATCH 1/1] vmscan: Support multiple kswapd threads per node

[Rik van Riel]

On Fri, 2020-10-02 at 09:03 +0200, Michal Hocko wrote:
> On Thu 01-10-20 18:18:10, Sebastiaan Meijer wrote:
> > (Apologies for messing up the mailing list thread, Gmail had fooled
> > me into
> > believing that it properly picked up the thread)
> > 
> > On Thu, 1 Oct 2020 at 14:30, Michal Hocko <mhocko@xxxxxxxx> wrote:
> > > On Wed 30-09-20 21:27:12, Sebastiaan Meijer wrote:
> > > > > yes it shows the bottleneck but it is quite artificial. Read
> > > > > data is
> > > > > usually processed and/or written back and that changes the
> > > > > picture a
> > > > > lot.
> > > > Apologies for reviving an ancient thread (and apologies in
> > > > advance for my lack
> > > > of knowledge on how mailing lists work), but I'd like to offer
> > > > up another
> > > > reason why merging this might be a good idea.
> > > > 
> > > > From what I understand, zswap runs its compression on the same
> > > > kswapd thread,
> > > > limiting it to a single thread for compression. Given enough
> > > > processing power,
> > > > zswap can get great throughput using heavier compression
> > > > algorithms like zstd,
> > > > but this is currently greatly limited by the lack of threading.
> > > 
> > > Isn't this a problem of the zswap implementation rather than
> > > general
> > > kswapd reclaim? Why zswap doesn't do the same as normal swap out
> > > in a
> > > context outside of the reclaim?

On systems with lots of very fast IO devices, we have
also seen kswapd take 100% CPU time without any zswap
in use.

This seems like a generic issue, though zswap does
manage to bring it out on lower end systems.

-- 
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