RFR (M): 8157952: Parallelize Memory Pretouch

Vitaly Davidovich vitalyd at gmail.com
Wed Sep 7 13:56:48 UTC 2016


On Wed, Sep 7, 2016 at 9:42 AM, Thomas Schatzl <thomas.schatzl at oracle.com>
wrote:

> Hi,
>
> On Wed, 2016-09-07 at 09:01 -0400, Vitaly Davidovich wrote:
> > Hi Thomas,
> >
> > On Wed, Sep 7, 2016 at 8:50 AM, Thomas Schatzl <thomas.schatzl at oracle
> > .com> wrote:
> > > Hi Vitaly,
> > >
> > > On Wed, 2016-09-07 at 10:54 +0000, Vitaly Davidovich wrote:
> > > > Hi Thomas,
> > > >
> > > > Why does the G1PretouchTask use an atomic ptr add to advance the
> > > > touch address?
> > >
> > >   to make sure that a particular thread gets a unique page to
> > > touch.
> > >
> > > Otherwise multiple threads might get the same. While this is not a
> > > correctness issue, it's useless work. Also the OS will likely let
> > > all but the first thread wait until the page has been allocated and
> > > cleared until they can proceed. I.e. the threads will be serialized
> > > again.
> > Oh, I thought each pretouch task is assigned a range of memory to
> > pretouch when its constructed, but apparently they all just "compete"
> > to claim the next chunk.  Not a big deal, but is there a reason you
> > chose to do it that way instead of assigning disjoint ranges to the
> > workers before starting the pretouch? Is it to avoid work imbalance
> > or something?
>
>   there is no particular reason, but imo it makes the code simpler by
> following existing patterns, and as you suggested automatically avoids
> imbalance.
>
> The synchronization required on the atomic add is (imo) negligible
> compared to pre-touching at least one GB of memory for every thread
> (i.e. the thread pre-clearing that memory on behalf of the OS).
>
Right, was just curious.

>
> Thanks,
>   Thomas
>
> Thanks
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