GC must ensure there is at least X% of free space in the heap after collection to avoid frequent collections

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Dmitry Olshansky (@DmitryOlshansky) reported this on 2015-09-18T14:43:10Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=15084

CC List

• Martin Nowak (@MartinNowak)
• Vladimir Panteleev (@CyberShadow)

Description

One thing that any remotely production-quality GC does is analyze the result of collection with respect to minimal headroom - X % (typically 30-50%). If we freed Y % of heap where Y < X, then the GC should extend the heap so that it get within X % mark of free space in the extended heap. 

See the exchange in NG discussion at http://forum.dlang.org/thread/mailman.149.1442256696.22025.digitalmars-d@puremagic.com

On 14-Sep-2015 21:47, H. S. Teoh via Digitalmars-d wrote:
> Over in the d.learn forum, somebody posted a question about poor
> performance in a text-parsing program. After a bit of profiling I
> discovered that reducing GC collection frequency (i.e., GC.disable()
> then manually call GC.collect() at some interval) improved program
> performance by about 20%.
>
...
On Monday, 14 September 2015 at 18:58:45 UTC, Adam D. Ruppe wrote: 
> Definitely. I think it hits a case where it is right at the edge of the line and you are allocating a small amount.
>
> So it is like the limit is 1,000 bytes. You are at 980 and ask it to allocate 30. So it runs a collection cycle, frees the 30 from the previous loop iteration, then allocates it again... so the whole loop, it is on the edge and runs very often.
>
> Of course, it has to scan everything to ensure it is safe to free those 30 bytes so the GC then runs way out of proportion.
>
> Maybe we can make the GC detect this somehow and bump up the size. I don't actually know the implementation that well though.

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dlang-bugzilla (@CyberShadow) commented on 2015-10-15T17:08:04Z

I thought our GC already did this, especially since the optimizations from this/last year. Martin?