Re: [PATCH] bcache: add cond_resched() in __bch_cache_cmp()

[Shile Zhang]

On 2019/3/7 23:44, Vojtech Pavlik wrote:
> On Thu, Mar 07, 2019 at 11:36:18PM +0800, Coly Li wrote:
> > On 2019/3/7 11:06 äå, Shile Zhang wrote:
> > > On 2019/3/7 18:34, Coly Li wrote:
> > > > On 2019/3/7 1:15 äå, shile.zhang@xxxxxxxxxxxxxxxxx wrote:
> > > > > From: Shile Zhang <shile.zhang@xxxxxxxxxxxxxxxxx>
> > > > >
> > > > > Read /sys/fs/bcache/<uuid>/cacheN/priority_stats can take very long
> > > > > time with huge cache after long run.
> > > > >
> > > > > Signed-off-by: Shile Zhang <shile.zhang@xxxxxxxxxxxxxxxxx>
> > > > Hi Shile,
> > > >
> > > > Do you test your change ? It will be helpful with more performance data
> > > > (what problem that you improved).
> > > In case of 960GB SSD cache device, once read of the 'priority_stats'
> > > costs about 600ms in our test environment.
> > After the fix, how much time it takes ?
> >
> >
> > > The perf tool shown that near 50% CPU time consumed by 'sort()', this
> > > means once sort will hold the CPU near 300ms.
> > >
> > > In our case, the statistics collector reads the 'priority_stats'
> > > periodically, it will trigger the schedule latency jitters of the
> > >
> > > task which shared same CPU core.
> > Hmm, it seems you just make the sort slower, and nothing more changes.
> > Am I right ?
> Well, it has to make the sort slower, but it'll also avoid hogging the
> CPU (on a non-preemptible kernel), avoiding a potential soft lockup
> warning and allowing other tasks to run.
Yes, there is a risk that other tasks have no chance to run due to sort 
hogging the CPU, it is harmful to some schedule-latency sensitive tasks.
This change just try to reduce the impact of sort, but not a performance 
improvement of it. I'm not sure if a better way can handle it more 
efficiency.

Thanks,

Shile