RFR (XS): 8138684: G1 decision about taking regions into the collection set is too aggressive

Jon Masamitsu jon.masamitsu at oracle.com
Thu Nov 12 19:41:14 UTC 2015



On 11/05/2015 02:49 AM, Thomas Schatzl wrote:
> Hi all,
>
>    can I have reviews for this small change to the policy that ends
> adding regions to the collection set?
>
> In some earlier review we noticed that G1 tries to fill up all remaining
> space completely.
>
> E.g. in
>
> 466 if ((2.0 * _sigma) * (double) bytes_to_copy > (double) free_bytes)
> {
> 467 // end condition 3: out-of-space (conservatively!)
> 468 }
>
> _sigma is .5 by default, and I have never seen anyone changing
> G1ConfidencePercent.
>
> This is a recipe for evacuation failure we want to avoid.
>
> So I thought of a formula that better represents the assumptions in the
> GC code, and came up with this:
>
>   487   double safety_factor = (100.0 / G1ConfidencePercent) * (100 +
> TargetPLABWastePct) / 100.0;
>   488   size_t expected_bytes_to_copy = safety_factor * bytes_to_copy;
>   489
>   490   if (expected_bytes_to_copy > free_bytes) {

Thomas,

Change looks good.

I would add a comment before the calculation of the safety factor.

// (100.0 / G1ConfidencePercent) represents a scale factor that
// allows a more conservative calculation of the expected bytes
// (i.e., a larger number expected bytes because there is always
// uncertainty in predictions).
// (100 + TargetPLABWastePct) / 100.0 represents the increase
// in expected bytes due to anticipated waste in the PLAB's due
// to unused space at the end of the evacuation.

If you find those accurate and helpful.

Jon

>
> I.e. the safety factor is first dependent on our target PLAB waste, and
> on the confidence of our predictions in general, scaling inversely (i.e.
> with high G1ConfidencePercent we assume that our assumption about
> keeping TargetPLABWastePct is more correct, and with lower
> G1ConfidencePercent we allow more slack).
>
> A (really) conservative buffer here is particularly interesting for
> dynamic IHOP, as it tends to try to push out marking rounds as long as
> possible.
>
> Since we are also in the process of investigating the policy surrounding
> mixed gcs and the predictions, this seems a lot better starting point
> too.
>
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8138684
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8138684/webrev
> Testing:
> jprt, benchmark runs with and without dIHOP without any real
> differences.
>
> Thanks,
>    Thomas
>
>



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