"--" at the beginning of a end-collection line and best region count when it's not 2048

[Thomas Schatzl]

Hi,

On Thu, 2017-11-16 at 09:15 -0800, Ron Reynolds wrote:
> -- 59G->59G(59G), 2.0932801 secs]
> ^^ this "--" is entirely undocumented as far as i can tell; i believe
> it indicates that the young collection (or any collection?) was
> cancelled (i've always seen these in the last-ditch-attempt young-
> collections before a full-GC).  is that correct?

I do not remember ever seeing that output, can you tell which VM and
which GC you are using? And maybe just a bit more of your log?

Potentially this is some leftover from some other log message printed
out concurrently, i.e. the log messages got intermixed.

> also the various docs recommend 2048 regions but if you MUST have
> more or less which should you choose and what are reasonable limits?
>  e.g., i have a 59G heap with default 16MB regions (which means there
> should be 3832 of them); in what cases (if any) would it make sense
> to use 32MB regions instead?  

If you have lots of humongous (large) objects, or as an option to
potentially decrease remembered set size (memory usage) and scan rs
time use larger regions. Another reason to increase heap region size
could be that 32M regions allow larger TLABs (for applications that
really burn through memory).

In most cases it does not matter.

See https://docs.oracle.com/javase/9/gctuning/garbage-first-garbage-col
lector-tuning.htm for some discussion. While this is for jdk9, almost
all of it applies to earlier versions too.

> most docs say to size your regions based on max-heap size rather than
> starting-heap size, which would seem to indicate <2048 regions is 

This changed in some 8 update or 9. Now defaults base the number of
regions based on both initial and max size.

> better than >2048, since at the start that hypothetical JVM would 

+-10% (or even more) generally do not make a difference. There are two
boundaries: since regions are units of evacuation, if the region is too
large the selection might impact the flexibility in selecting which
regions to evacuate (i.e. if any single region G1 may evacuate blows
the pause time goal).

I *think* 2048 is just a random number far larger than maybe a (low)
few hundred where G1 might get into trouble with finding consecutive
free regions for large objects easily. However at the same time with
larger regions there will be less objects that qualify as large, so it
depends.

G1 needs at least one eden, one survivor and one old region to operate.

> certainly have fewer than 2048 regions until it grew into its max-
> heap size.

Thanks,
  Thomas