linux-kernel - Re: [PATCH 2/3] bcache: Convert to lib/time

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Message-ID: <20220831165437.l27raas6k5nlqsdg@moria.home.lan>
Date:   Wed, 31 Aug 2022 12:54:37 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Coly Li <colyli@...e.de>
Cc:     linux-kernel@...r.kernel.org, linux-bcache@...r.kernel.org
Subject: Re: [PATCH 2/3] bcache: Convert to lib/time_stats

On Thu, Sep 01, 2022 at 12:00:17AM +0800, Coly Li wrote:
> 
> 
> > 2022年8月30日 00:53，Kent Overstreet <kent.overstreet@...ux.dev> 写道：
> > 
> > This patch converts bcache to the new generic time_stats code
> > lib/time_stats.c. The new code is from bcachefs, and has some changes
> > from the version in bcache:
> > 
> > - we now use ktime_get_ns(), not local_clock(). When the code was
> >   originally written multi processor systems that lacked synchronized
> >   TSCs were still common, and so local_clock() was much cheaper than
> >   sched_clock() (though not necessarily fully accurate, due to TSC
> >   drift). ktime_get_ns() should be cheap enough on all common hardware
> >   now, and more standard/correct.
> > 
> > - time_stats are now exported in a single file in sysfs, which means we
> >   can improve the statistics we keep track of without changing all
> >   users. This also means we don't have to manually specify which units
> >   (ms, us, ns) a given time_stats should be printed in; that's handled
> >   dynamically.
> > 
> > - There's a lazily-allocated percpu buffer, which now needs to be freed
> >   with time_stats_exit().
> > 
> > Signed-off-by: Kent Overstreet <kent.overstreet@...ux.dev>
> > Cc: Coly Li <colyli@...e.de>
> 
> Hi Kent,
> 
> Overall I am OK with the change to bcache code. You may add
> 	Acked-by: Coly Li <colyli@...e.de>
> in future version of this patch.
> 
> In bcache-tools, they don’t read the changed sysfs files (including bcache-status), IMHO changing the output format won’t be problem for upstream.
> 
> My only question is, how to understand the time_stats_to_text() output format,
> count:          3
> rate:           0/sec
> frequency:      4 sec
> avg duration:   4 sec
> max duration:   4 sec
> quantiles (ns): 0 4288669120 4288669120 5360836048 5360836048 5360836048 5360836048 5360836048 5360836048 5360836048 5360836048 5360836048 5360836048 5360836048 5360836048
> 
> Fro the above output, what are “rate”, “frequence” and “quantiles” for?

Rate and frequency are inverses - in this example, we're seeing 4 events per
second.

The quantiles are for the duration, they give you an idea of the statistical
distribution, see https://en.wikipedia.org/wiki/Quantile

In the near future, the quantiles will be removed and replaced with standard
deviation, true and weighted - the quantiles algorithm isn't super accurate, we
can give more accurate numbers with standard deviation.