[Devel] [TRD] optimization for zero-filled pages written by VMs

Roman Kagan rkagan at virtuozzo.com
Fri Sep 8 11:04:39 MSK 2017

On Thu, Sep 07, 2017 at 04:41:11PM +0300, Konstantin Khorenko wrote:
> 1. Feature
> Optimization for zero-filled pages written by VMs
> https://jira.sw.ru/browse/PSBM-64658
> 2. Description
> Any page before being written to the physical swap is checked if it is filled
> by zeroes and mapped to a zero page in this case instead of actual swap out.
> The feature is implemented on the tswap level, so the page is checked for zeroes
> before putting to tswap.

1) why is it an improvement over KSM?

2) IIUC zero detection happens at reclaim time.  Isn't it too late?
   Shouldn't the goal be to avoid triggering reclaim at all?

3) doesn't the regular Linux reclaimer test for zeroes?

4) (and what is tswap BTW?)

> Usecase when the feature is expected to provide most performance boost: many
> Windows-based VMs started simultaneously on a Hardware Node with overcommit
> => on any restart of an overcommitted HN which runs Windows-based VMs.
> Windows-based VMs are chosen here because Windows is known to rewrite its RAM
> with zeroes on boot and lazily while it is running.


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