jjs and the tiny heap that couldn't

[Sundararajan Athijegannathan]

I don't think jjs was ever GC tuned! Most likely default copied/adapted
from somewhere else!

Please file a bug with your findings.

PS. jjs of jdk9 is very different (in interactive mode) from that of
jdk8u. jdk9 versions on jline+many other desktop stuff and so would load
lot more code.

Thanks,

-Sundar


On 8/26/2016 5:54 AM, Claes Redestad wrote:
> Hi,
>
> as I was playing around with jjs to get some data points on recent indy
> bootstrap improvements when I noticed that the default heap size might
> be excessively small (linux x64 on a machine with 64G RAM):
>
> exit.js:
> exit();
>
> jdk9/bin/jjs -J-Xlog:gc exit.js
> [0.026s][info][gc] Using G1
> [0,246s][info][gc] GC(0) Pause Young (G1 Evacuation Pause) 3M->0M(8M)
> (0,243s, 0,246s) 3,643ms
> [0,321s][info][gc] GC(1) Pause Young (G1 Evacuation Pause) 2M->1M(8M)
> (0,314s, 0,321s) 6,996ms
> [0,353s][info][gc] GC(2) Pause Young (G1 Evacuation Pause) 2M->1M(8M)
> (0,349s, 0,353s) 3,307ms
> [0,485s][info][gc] GC(3) Pause Young (G1 Evacuation Pause) 3M->1M(8M)
> (0,474s, 0,485s) 10,991ms
> [0,581s][info][gc] GC(4) Pause Young (G1 Evacuation Pause) 3M->2M(14M)
> (0,573s, 0,581s) 7,879ms
> [0,743s][info][gc] GC(5) Pause Young (G1 Evacuation Pause) 6M->2M(14M)
> (0,735s, 0,743s) 8,678ms
>
> It appears jjs starts out with a tiny heap, leading to substantial
> time spent in GC just to bootstrap.
>
> A small bump to -Xms35m removes GC activity during bootstrap (for
> reference on this machine java defaults to an -Xms around 140m):
>
> jdk9/bin/jjs -J-Xmx35m -J-Xlog:gc exit.js
> [0.026s][info][gc] Using G1
>
> The story is similar on 8u, but there I had to increase it all the way
> to -Xms100m to get GC activity down to zero.
>
> Is it just me, or does it seem odd that jjs is so aggressively tuned
> for minimal footprint that we spend considerable time in GC just to get
> up and running - and still end up increasing the heap size in the
> process?
>
> Thanks!
>
> /Claes