Degenerated GC pauses for 5-10 seconds

[Aleksey Shipilev]

Hi,

On 1/11/19 3:03 PM, Alexander Yakushev wrote:
> *Setup: *
> - openjdk-shenandoah-jdk8-latest-linux-x86_64-release from
> builds.shipilev.net, downloaded on 2018-12-31.
> - -Xmx55g -Xms55g -XX:+UseShenandoahGC -XX:+AlwaysPreTouch

Ok, this is recent enough.

>   // Also, not sure if relevant:
>   -XX:+UnlockDiagnosticVMOptions -XX:+DebugNonSafepoints

This does not seem relevant.

> *Problem:*
> Shenandoah does an excellent job dealing with ~4GB/sec allocation rate on
> a 55GB heap staying under 100ms max pause time and ~500ms max cycle time.
> However,  on particularly nasty spikes, it drops into the degenerated mode
> which causes a 5-10 second STW pause. So far I observed that three times,
> each with a huge pause. There doesn't seem to be other "normal"
> degenerated cycles, so I assume every degenerate cycle causes a pause like
> that.

Here's the excerpt, verbatim:

[Concurrent reset
Using 25 of 25 workers for concurrent reset
Failed to allocate 2048K
Cancelling GC: Allocation Failure
 46G->52G(55G), 11069.097 ms]

So this is not the pause per se. It looks like cancellation triggered when "Concurrent reset", the
first action in the cycle, was running. Why was it running for 11 seconds is not clear right now,
let me look around.

Can you grep your other logs and see if those 5-10 second "pauses" were also "Concurrent reset"s?

Cheers,
-Aleksey