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

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Message-Id: <1F91055C-4253-42CD-8A4A-8B8EA2CF1D6E@suse.de>
Date:   Thu, 1 Sep 2022 01:02:37 +0800
From:   Coly Li <colyli@...e.de>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
Cc:     linux-kernel@...r.kernel.org, linux-bcache@...r.kernel.org
Subject: Re: [PATCH 2/3] bcache: Convert to lib/time_stats



> 2022年9月1日 00:54，Kent Overstreet <kent.overstreet@...ux.dev> 写道：
> 
> 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.
> 

Can I understand that rate = 1/frequency ?  Then frequency 4 is around to rate 0.

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

I wanted to ask how to read the quantiles line. Does it mean that 1 ns is equally divided by 15 segments, and the counter values are for the divided 1/15 ns segments?

> 
> 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.

Quantiles are much humane, standard deviation is scared… Just FYI LOL.

Thanks.

Coly Li