Shenandoah experimental flags

[Aleksey Shipilev]

On 5/1/20 4:36 PM, Aleksey Shipilev wrote:
> On 4/30/20 11:17 PM, Cristian Velazquez wrote:
>> Thanks for the prompt response. I'm using the docker jdk8 image (shipilev/openjdk-shenandoah:8).
> 
> Aha! I am able to reproduce it with current sh/jdk8. Let me see what is up there.

I believe it is what I initially expected: concurrent mark runs into termination very fast, the
periodic task that flushes SATB buffers does not have the opportunity to act, then final mark hits
and discovers lots of hidden work from the SATB buffers.

A new thing I see is, the SATB traffic itself is too low to enqueue SATB buffers naturally. Indeed,
reducing SATB buffer sizes is the workaround here.

It is interesting that it happens reliably on sh/jdk8, but not on sh/jdk11 and later. Maybe Jetty is
using something unusual, like lots of phantom references? Anyhow, another way to dodge this problem
is upgrading.

As for sh/jdk8, we should probably try and see if we can adjust SATB sizes looking back at traffic
we are getting. The nuclear option would be to introduce another pause where we force all threads go
give up their SATB buffers, regardless of their capacity.

That is something to look at next week.

With some instrumentation, good cycle:

[Concurrent marking, start]
    Using 4 of 8 workers for concurrent marking
Flushed SATB buffer for thread qtp475266352-44
Flushed SATB buffer for thread qtp475266352-30
Flushed SATB buffer for thread qtp475266352-31
Flushed SATB buffer for thread qtp475266352-48
Flushed SATB buffer for thread qtp475266352-61
Flushed SATB buffer for thread qtp475266352-47
Flushed SATB buffer for thread qtp475266352-63
Flushed SATB buffer for thread qtp475266352-21-selector-ServerConnectorManager at 1c07660a/0
Flushed SATB buffer for thread qtp475266352-20-selector-ServerConnectorManager at 1c07660a/2
Flushed SATB buffer for thread qtp475266352-22-selector-ServerConnectorManager at 1c07660a/3
Flushed SATB buffer for thread qtp475266352-27-selector-ServerConnectorManager at 1c07660a/1
Flushed SATB buffer for thread qtp475266352-26
Flushed SATB buffer for thread qtp475266352-37
[Concurrent marking, 911.426 ms]
[Pause Final Mark, start]
    Using 8 of 8 workers for final marking
    Adaptive CSet Selection. Target Free: 290M, Actual Free: 316M, Max CSet: 87381K, Min Garbage: 0B
    Collectable Garbage: 956M (97%), Immediate: 1024K (0%), CSet: 955M (97%)
    Pacer for Evacuation. Used CSet: 965M, Free: 247M, Non-Taxable: 25368K, Alloc Tax Rate: 9.5x
[Pause Final Mark, 0.917 ms]

...and bad cycle:

[Concurrent marking, start]
    Using 4 of 8 workers for concurrent marking
[Concurrent marking, 1.937 ms]
[Pause Final Mark, start]
    Using 8 of 8 workers for final marking
    Adaptive CSet Selection. Target Free: 290M, Actual Free: 456M, Max CSet: 87381K, Min Garbage: 0B
    Collectable Garbage: 918M (96%), Immediate: 0B (0%), CSet: 918M (96%)
    Pacer for Evacuation. Used CSet: 926M, Free: 382M, Non-Taxable: 39184K, Alloc Tax Rate: 5.9x
[Pause Final Mark, 987.943 ms]

-- 
Thanks,
-Aleksey