RFR (M): 8157952: Parallelize Memory Pretouch

[Thomas Schatzl]

Hi Jon,

On Wed, 2016-09-07 at 14:37 -0700, Jon Masamitsu wrote:
> Thomas,
> http://cr.openjdk.java.net/~tschatzl/8157952/webrev/src/share/vm/gc/g
> 1/g1_globals.hpp.frames.html
> Could we change G1PreTouchChunkSize to PreTouchChunkSize?  It
> seems like something we would want to do for other GC's at some
> point.

Can do. Maybe "ParallelPreTouchChunkSize"?

> http://cr.openjdk.java.net/~tschatzl/8157952/webrev/src/share/vm/gc/s
> hared/workgroup.hpp.frames.html
> 
>  64     _gc_id(Universe::is_fully_initialized() ? GCId::current_raw()
> : 0)
> What prompted the above change?

The problem is that we need to use a work gang during initialization of
the VM. GCId and the whole logging framework is not up at this point,
and GCId::current_raw() returns -1 (invalid) that is checked in some
asserts.

That is also why the code only logs messages when we are not
initializing the heap.

I will see if there is another, more clean, way to enable the workgang
framework during VM initialization.

> http://cr.openjdk.java.net/~tschatzl/8157952/webrev/src/share/vm/gc/g
> 1/g1PageBasedVirtualSpace.cpp.frames.html
> 
> I don't think there is anything G1 specific about class
> G1PretouchTask.  Could it be renamed PretouchTask and put into its
> own file for easier reuse?  If you think that is premature, ignore
> this comment.

I do think that is premature.

Also, there will be need for a special version for parallel gc as it
does not use the WorkGang framework... :(

I am not sure we will need it for CMS at this time.

> Pretouch() allows for the case where the work is done by the 
> current thread (only 1 chunk or null pretouch_gang).
>   259 void G1PageBasedVirtualSpace::pretouch(size_t start_page,
> size_t size_in_pages, WorkGang* pretouch_gang) {
> Pretouching is so slow that I would not think it made a difference if
> parallel worker were used instead of the current thread.  Simpler to
> just always require a gang and use 1 worker in the degenerate case?

Can do.

> 
> Rest looks good.

I will provide an updated webrev later.

Thanks,
  Thomas