RFR: Shortcut concurrent cycle when enough immediate garbage is reclaimed

[Zhengyu Gu]

Good to me.

-Zhengyu

On 08/03/2017 10:20 AM, Aleksey Shipilev wrote:
> Meditating over performance data from our workloads, it struck me that we miss an obvious
> optimization opportunity. In our final mark, we know the liveness more or less precisely
> (conservatism is coming from SATB, but we don't care about it here). We exploit this by figuring out
> the immediate garbage regions, and reclaiming them on the spot, without adding them to collection set.
>
> Now, in many workloads, especially those following the generational hypothesis to the extreme, we
> have lots and lots of immediately reclaimed regions (E.g. various SPECjvm workloads have around
> 70..95% of regions immediately reclaimable). We can exploit this further by shortcutting the
> concurrent cycle when we already have a huge bang from immediate regions already.
>
> In other words, if there is enough immediately reclaimed regions, skip concurrent evac and
> update-refs. This saves cycles walking the heap in update-refs, saves pauses for update-refs, saves
> enabling the write barriers and going to slow-paths for cset checks in WBs, etc.
>
> Webrev:
>  http://cr.openjdk.java.net/~shade/shenandoah/immediate-shortcut/webrev.01/
>
> Example run with XmlTransform (has ~92% immediately reclaimable regions) -- cuts total pause time
> ~2.3x, the number of pauses ~1.7x, etc:
>  http://cr.openjdk.java.net/~shade/shenandoah/immediate-shortcut/xmltransform-before.txt
>  http://cr.openjdk.java.net/~shade/shenandoah/immediate-shortcut/xmltransform-after.txt
>
> Testing: hotspot_gc_shenandoah
>
> Thanks,
> -Aleksey
>