I just created a new branch gc_ms2_tuning. It contains a port of the dynamic GC threshold to the new GC code. The memory_used counter now also accounts for attribute and other fixed size storage and ignores constant objects. The old code could lead to a lot more memory being used than accounted for, so rather low thresholds could still work very well. To address this, the default dynamic GC threshold has been increased to 75%. Note that these changes have not been applied to the old GC. It still uses the old accounting.
It also turned out that the double linked lists in the new GC code are quite a hit on performance and memory consumption. I replaced them with a simple array as GC mark stack but afaics the double linked lists will also be used for Generational GC. So maybe this approach doesn't work out.
Here are some more notes on the GC thresholds to avoid confusion. There are two thresholds: a static and a dynamic one. The Parrot memory management counts the amount of memory newly allocated since the previous GC run. If this amount exceeds both the static and dynamic threshold, another GC run is triggered.
Currently, the static threshold of the old GC (ms) is hardcoded to 1MB. The static threshold of the new GC (ms2) is 256MB in trunk and 1MB in my new branch.
The dynamic threshold is a percentage of the total memory used directly after the last GC. (After a GC run we know exactly the minimum memory a process requires.) This allows us to adjust the timing of GC runs to the actual memory requirements of a process. It also roughly limits the percentage of memory wasted. Currently, the dynamic threshold of the old GC is 25% (although this is skewed, see my notes above). The new GC in trunk has no dynamic threshold, the new GC in my new branch uses 75%.
The dynamic threshold is a space-time tradeoff. Generally, lower values cause more GC runs, more execution time and less memory consumption. But on memory limited machines, a lower threshold can also improve general perfomance if it avoids swapping.
Nick _______________________________________________ http://lists.parrot.org/mailman/listinfo/parrot-dev
