[15] RFR 8243564: Shenandoah: Add degen entry after concurrent weak roots

[Zhengyu Gu]

On 4/24/20 2:42 PM, Aleksey Shipilev wrote:
> On 4/24/20 6:50 PM, Zhengyu Gu wrote:
>> When Shenandoah encounters OOM during concurrent weak roots, it is
>> better and cheaper to enter degenerated GC and perform STW class
>> unloading than concurrent class unloading.
>>
>>
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8243564
>> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8243564/webrev.00/index.html
> 
> This looks good, except a few nits:
> 
>   *) Shouldn't we still drop the flags in ShenandoahMarkCompact::do_it? The code in full_gc_prepare
> block below makes a point to drop all the flags when going into Full GC.
> 

_degenerated_weak_roots is the only entry point that carries flags, we 
cleanup those flags there.

>   *) In op_degenerated here, shouldn't we check for is_concurrent_unloading_in_progress instead?
>   1948       if (!has_forwarded_objects() &&
> ShenandoahConcurrentRoots::can_do_concurrent_class_unloading()) {

We are trying to disarm nmethods that armed for concurrent marking. If 
we get to here due to degenerated_mark, 
is_concurrent_unloading_in_progress flag may not be set, but we still 
need to disarm nmethods here.

> 
>   *) In op_degenerated, I think the point was to call op_*(). Why can't we call
> op_conc_class_unloading() and remove this whole block?
> 
> 1957       // No fallthrough
> 1958       if (point == _degenerated_weak_roots) {
> 1959         // Perform STW class unloading
> 1960         if (is_concurrent_class_unloading_in_progress()) {
> 1961           stw_unload_classes(false /* full gc*/);
> 1962           set_concurrent_class_unloading_in_progress(false);
> 1963         }
> 1964
> 1965         // cleanup flags that may be carried into degenerated GC
> 1966         set_concurrent_weak_root_in_progress(false);
> 1967         set_concurrent_strong_root_in_progress(false);
> 1968       }
> 1969
> 

concurrent version is more expensive than STW version (different 
implementation)

-Zhengyu