[linux-lvm] Discussion: performance issue on event activation mode

[heming.zhao at suse.com]

On 6/7/21 6:27 PM, Martin Wilck wrote:
> On So, 2021-06-06 at 11:35 -0500, Roger Heflin wrote:
>> This might be a simpler way to control the number of threads at the
>> same time.
>>
>> On large machines (cpu wise, memory wise and disk wise).   I have
>> only seen lvm timeout when udev_children is set to default.   The
>> default seems to be set wrong, and the default seemed to be tuned for
>> a case where a large number of the disks on the machine were going to
>> be timing out (or otherwise really really slow), so to support this
>> case a huge number of threads was required..    I found that with it
>> set to default on a close to 100 core machine that udev got about 87
>> minutes of time during the boot up (about 2 minutes).  Changing the
>> number of children to =4 resulted in udev getting around 2-3 minutes
>> in the same window, and actually resulted in a much faster boot up
>> and a much more reliable boot up (no timeouts).
> 
> Wow, setting the number of children to 4 is pretty radical. We decrease
> this parameter often on large machines, but we never went all the way
> down to a single-digit number. If that's really necessary under
> whatever circumstances, it's clear evidence of udev's deficiencies.
> 
> I am not sure if it's better than Heming's suggestion though. It would
> affect every device in the system. It wouldn't even be possible to
> process more than 4 totally different events at the same time.
> 

hello

I tested udev.children_max with value 1, 2 & 4. The results showed it
didn't take effect, and the booting time even longer than before.
This solution may suite for some special cases.

(my env: kvm-qemu vm, 6vpu, 22G mem, 1015 disks)

Regards
heming