On Mon, Jan 8, 2018 at 10:26 AM, Mark Thomas <ma...@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
