[Gc] Problems with GC performance using gcj (Repost text only)

Bruce Hoult bruce at hoult.org
Tue Jan 11 16:30:48 PST 2011


On Wed, Jan 12, 2011 at 4:58 AM, Ben Keppler <bkeppler at tridentms.com> wrote:
> NOTE:  Reposting as using text only.
> NOTE:  This question was also posted on the gcj mailing list.
>
> We are using gcj for a time sensitive application.  One of the requirements of the application is that messages be transmitted within a 60ms timeframe.  Unfortunately, the Boehm GC used in gcj is not generational and thus every collection is of the "stop the world" variety.  We are observing (using "GC_PRINT_STATS") regular collections that stop the world for periods in the 400ms range.  This is a problem for us.

How big is your heap? And what is the CPU?

I'm seeing pause times of ~40 mS in a 75 MB heap on the d2c Dylan
compiler on my Core i7 @3.44 GHz. That with a pure stop-the-world
setup.

p.s. parallel-mark makes it *slower*, no matter whether I use anything
between 2 and 8 threads, so that is off

p.s.2. I actually get about 20% more throughput by preallocating the
heap to around 256 - 512 MB. There are still a few GCs but a lot fewer
and they still take ~40 mS even in the larger heap (the amount of live
data is the same).  A typical compile for my test program allocates
around 1.4 GB.  Disabling GCs/using GC_malloc_uncollectable, or using
malloc(), or even a simple bump-the-ptr malloc is slower than using
GC_malloc -- at some point it takes more time to create VM space than
to GC what you already have.



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