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From Leon Rosenberg <rosenberg.l...@gmail.com>
Subject Re: GC allocation failure
Date Tue, 09 Jan 2018 05:23:59 GMT
On Mon, Jan 8, 2018 at 10:26 AM, Mark Thomas <markt@apache.org> wrote:

> On 08/01/18 15:16, Christopher Schultz wrote:
>
> <snip/>
>
> >> Therefore, the first time that the GC runs, the process can take
> >> longer. Also, the heap is more likely to be fragmented and require
> >> a heap compaction. To avoid that, till now my strategy is to: -
> >> Start application with the minimum heap size that application
> >> requires - When the GC starts up, it runs frequently and
> >> efficiently because the heap is small
> >
> > I think this is a reasonable expectation for someone who doesn't
> > understand the Black Art of Garbage Collection, but I'm not sure it's
> > actually true. I'm not claiming that I know any better than you do,
> > but I suspect that the collector takes its parameters very seriously,
> > and when you introduce artificial constraints (such as a smaller
> > minimum heap size), the GC will attempt to respect those constraints.
> > The reality is that those constraints are completely unnecessary; you
> > have only imposed them because you think you know better than the GC
> > algorithm.
>
> Generally, the more memory available, the more efficient GC is. The
> general rule is you can optimise for any two of the following at the
> expense of the third:
> - low pause time
> - high throughout
> - low memory usage
>
> It has been a few years since I listened to the experts talk about it
> but a good rule of thumb used to be that you should size your heap 3-5
> times bigger than the minimum heap used once the application memory
> usages reaches steady state (i.e. the minimum value of the sawtooth on
> the heap usage graph)
>
>
Actually G1, which is very usable with java8 and default in jdk9, doesn't
produce the sawtooth graph anymore.
I also think the amount of memory has less influence on GC Performance in
G1 or Shenandoah, but instead influence if they would perform a STW phase
(which of course is also performance related, but differently).
But I am not an expert either, so I might be wrong here.

As for OP's original statement: "When the GC starts up, it runs frequently
and
efficiently because the heap is small", I don't think it is correct
anymore, especially not for G1, as long as the object size is reasonable
(not Humongous).


Leon

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