Re: Scrub of my nvme SSD has slowed by about 2/3

[Martin Steigerwald <martin@lichtvoll.de>]

Qu Wenruo - 11.07.23, 13:33:50 CEST:
> >> BTW, what's the CPU usage of v6.3 kernel? Is it higher or lower?
> >> And what about the latency?
> > 
> > CPU usage is between 600-700% on 6.3.9, Latency between 50-70 µs.
> > And
> > scrubbing speed is above 2 GiB/s, peaking at 2,27 GiB/s. Later it
> > went down a bit to 1,7 GiB/s, maybe due to background activity.
> 
> That 600~700% means btrfs is taking all its available thread_pool
> (min(nr_cpu + 2, 8)).
> 
> So although the patch doesn't work as expected, we're still limited by
> the csum verification part.
> 
> At least I have some clue now.

Well it would have an additional 800-900% of CPU time left over to use 
on this machine, those modern processors are crazy. But for that it 
would have to use more threads. However if you can make this more 
efficient CPU time wise… all the better.

> > I'd say the CPU usage to result (=scrubbing speed) ratio is much,
> > much better with 6.3. However the latencies during scrubbing are
> > pretty much the same. I even seen up to 0.2 ms.
[…]
> >> If you're interested in more testing, you can apply the following
> >> small diff, which changed the batch number of scrub.
[…]
> > No time for further testing at the moment. Maybe at a later time.
> > 
> > It might be good you put together a test setup yourself. Any
[…]
> Sure, I'll prepare a dedicated machine for this.
> 
> Thanks for all your effort!

You are welcome.

Thanks,
-- 
Martin