[systemd-devel] udevd: increase maximum number of children

[Harald Hoyer]

On 28.11.2014 13:59, Robert Milasan wrote:
> On Fri, 28 Nov 2014 13:51:04 +0100
> "Tom Gundersen" <teg at jklm.no> wrote:
>  
>>> Also the patch changes the logging level of 'maximum number of
>>> children reached' to an error, this should be visible as an error
>>> when the number has been reached.
>>
>> I don't think an error is appropriate here (as nothing actually
>> fails). At most a warning I would say.
>>
>> Cheers,
>>
>> Tom
>>
> 
> This is an error, because the behavior makes the system not work
> correctly.
> 
> I don't care about a warning that much, but in this case and reference
> bug, we see a bug and/or an error which is causes by the small amount of
> children, or the impossibility of udev daemon to create new
> children/workers, stopping the queue processing until the number of
> children is lower the children_max.
> 
> Anyway, please do as you wish as long as it gets fixed.
> 


This is not true. It only defers the uevent until a worker is available. So
logging it as an error is incorrect. It's a debug message.

We don't have unlimited resources. You don't do "make -j" with unlimited make
jobs either for a kernel build to get the minimum build time.

Having 1024 concurrent blkid's running will slow down a machine significantly,
because concurrent I/O over a single path is never good.

So, what we see is a lot of I/O errors because of timeouts on huge systems, if
the bottleneck is saturated.

Ideally we would limit the concurrent I/O, but we can't know (in a simple way)
where the bottleneck is. It might be a SAN behind FCoE, or iscsi and the
network is the bottleneck.

That being said, I don't have a sane solution to satisfy everybody.