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Re: [PATCH v3 7/7] introduce simple linear scan rate limiting mechanism

From: Peter Xu
Subject: Re: [PATCH v3 7/7] introduce simple linear scan rate limiting mechanism
Date: Thu, 19 Nov 2020 15:02:50 -0500

On Thu, Nov 19, 2020 at 03:59:40PM +0300, Andrey Gruzdev wrote:
> Since reading UFFD events and saving paged data are performed
> from the same thread, write fault latencies are sensitive to
> migration stream stalls. Limiting total page saving rate is a
> method to reduce amount of noticiable fault resolution latencies.
> Migration bandwidth limiting is achieved via noticing cases of
> out-of-threshold write fault latencies and temporarily disabling
> (strictly speaking, severely throttling) saving non-faulting pages.

Just curious: have you measured aver/max latency of wr-protected page requests,
or better, even its distribution?

I believe it should also be relevant to where the snapshot is stored, say, the
backend disk of your tests.  Is that a file on some fs?

I would expect the latency should be still good if e.g. the throughput of the
backend file system is decent even without a patch like this, but I might have
missed something..

In all cases, it would be very nice if this patch can have the histogram or
aver or max latency measured and compared before/after this patch applied.


Peter Xu

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