Why does load average on host increases as Allocation Stall happens?

Florian Weimer fw at deneb.enyo.de
Thu Nov 14 19:24:41 UTC 2019

* Sundara Mohan M.:

>    I have notices Load average on the host increases 5 - 10 times
> when Allocation Stall happens, trying to understand what causes load
> average to increase when this happens.

> 2. Since load average increase matches exactly with allocation stall, is
> there any other operation (like Flushing page)  can cause this behavior?

I don't know ZGC internals, but I think it stalls the application when
the GC cannot keep up.  This is a last resort.  Before that happens,
more GC threads will try hard to reclaim memory.  That work increases
system load.

An alternative explanation could be that something else consumes CPU
resources, taking it away from the GC threads, so that they cannot
keep up, and ZGC has to introduce allocation stalls.

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