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SubjectRe: [PATCH 2/3] bcache: Convert to lib/time_stats
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> 2022年9月1日 00:54,Kent Overstreet <kent.overstreet@linux.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@linux.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@linux.dev>
>>> Cc: Coly Li <colyli@suse.de>
>>
>> Hi Kent,
>>
>> Overall I am OK with the change to bcache code. You may add
>> Acked-by: Coly Li <colyli@suse.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

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