RE: Avoid function call with JVMPI enabled

["Boehm, Hans" <hans dot boehm at hp dot com>]

Doesn't it make sense to turn this off by default?  Even checking
a global flag here adds overhead.  And the JVMPI implementation,
at least for allocation, is currently pretty bogus.

I looked at the JVMPI spec again.  It does require that GC be disabled
during the call-back for an allocation.  But I think its notion of 
having the GC disabled really corresponds to acquiring and holding
the allocation lock, not causing the collector to expand the heap
when it would otherwise collect.  We currently do the latter, which
is bound to result in unbounded heap growth eventually.  (The spec
seems unclear about this, though it's an important distinction.
It's not clear either solution is acceptable, but they fail very
differently.)

Even if this were all fixed, it seems to me that the JVMPI allocation
interface has enough inherent overhead that I wouldn't trust a profiled
app to behave anything like the original, especially since we just
added a heavily contended lock.  Maybe this won't affect allocation
behavior for most apps.  But that's not at all clear.  It can
certainly slow things down tremendously.

Hans

> -----Original Message-----
> From: java-patches-owner@gcc.gnu.org
> [mailto:java-patches-owner@gcc.gnu.org]On Behalf Of Bryce McKinlay
> Sent: Monday, August 23, 2004 2:31 PM
> To: Boehm, Hans
> Cc: 'java-patches@gcc.gnu.org'
> Subject: FYI: Avoid function call with JVMPI enabled
> 
> 
> Boehm, Hans wrote:
> >I always build with JVMPI disabled, since it does bad things
> >to the allocation sequence.  (It adds a third (!) function call
> >to every allocation call.  Unless of course you actually turn
> >it on, in which case it's MUCH worse.)
> >  
> >
> I hadn't noticed this previously. It turns out that the test 
> to check if 
> JVMPI is enabled was not being inlined before calling 
> jvmpi_notify_alloc. This patch fixes this by doing the test 
> in a macro. 
> The macro ensures that the jvmpi_notify_alloc call is not made unless 
> JVMPI is actually enabled.
> I'm checking this in.
> Bryce
>