Hello Houkun,
There was patch https://review.whamcloud.com/34400 
"LU-947<https://jira.whamcloud.com/browse/LU-947> ptlrpc: allow stopping 
threads above threads_max" landed for the 2.13 release. You could apply this 
patch to your 2.12 release, or test with 2.14.0. Note that this patch only 
lazily stops threads as they become idle, so there is no guarantee that they 
will all stop immediately when the parameter is changed. It may be some time 
and processed RPCs before the higher-numbered threads exit.

It might be possible to wake up all of the threads when the threads_max 
parameter is reduced, to have them check for this condition and exit. However, 
this is a very unlikely condition under normal usage.

I would recommend to test with increasing the thread count, rather than 
decreasing it...

Cheers, Andreas

On Sep 20, 2021, at 02:29, Houkun Zhu via lustre-discuss 
<[email protected]> wrote:


Hi guys,

I’m creating an automatic lustre performance tuning system. But I find it’s 
hard to tune parameter regarding  max service threads because it seems there is 
only guarantee of max threads when we increase the parameter. I’ve found a 
similar 
discussion<http://lists.lustre.org/htdig.cgi/lustre-discuss-lustre.org/2011-August/010358.html>
 from 2011, is there any updates?

Though I can wait for the number threads to automatically decrease, I didn’t 
find ways which can really indicate the current running threads. I’ve tried 
thread_started (e.g., lctl get_param mds.MDS.mdt.threads_,started). But this 
param doesn’t change.

Looking forward to your help! Thank you in advance!

Best regards,
Houkun

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