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Hi all,<br>
<br>
Sending this to both runtime and GC since I think it concerns both
areas.<br>
<br>
I'd like some feedback on this preliminary change. I still want to
do some more testing and evaluation before I ask for final reviews:<br>
<br>
<a class="moz-txt-link-freetext" href="http://cr.openjdk.java.net/~brutisso/7197666/webrev.00/">http://cr.openjdk.java.net/~brutisso/7197666/webrev.00/</a><br>
<br>
In particular I would like some feedback on these questions:<br>
<br>
- I am adding a flag that has the same value on all platforms except
Solaris x86. There is the product_pd flag macro to support this. But
there is no experimental_pd marcro. I would have preferred to make
my new flag experimental. Should I add experimental_pd or should I
just use a product flag?<br>
<br>
- Even with product_pd I think I still have to go in to all the
different platform files and add the exact same code to give the
flag a default value on all platforms. Is there a way to have a
default value and only override it on Solaris x86?<br>
<br>
- The class I am adding,
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ArrayAllocator, wants to choose between doing malloc and mmap.
Normally we use ReservedSpace and VirtualSpace to get mapped memory.
However, those classes are kind of clumsy when I just want to
allocate one chunk of memory. It is much simpler to use the
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os::reserve_memory() and os::commit_memory() methods directly. I
think my use case here motivate using these methods directly, but is
there some reason not to do that?<br>
<br>
Some background on the change:<br>
<br>
The default implementation of malloc on Solaris has several
limitation compared to malloc on other platforms. One limitation is
that it can only use one consecutive chunk of memory. Another
limitation is that it always allocates in this single chunk of
memory no matter how large the requested amount of memory is. Other
malloc implementations normally use mapped memory for large
allocations.<br>
<br>
The Java heap is mapped in memory and we try to pick a good address
for it. The lowest allowed address is controlled by
HeapBaseMinAddress. This is only 256 MB on Solaris x86 (other
platforms have at least 2 GB). Since the C heap ends up below the
Java heap it means that in some cases it is limited to 256 MB.<br>
<br>
When we run with ParallelOldGC we get three task queues per GC
thread. Each task queue takes mallocs 1MB. The failing machine in
the bug report has lots of CPUs and ends up with 83 GC threads. This
is 249 MB, which is more than we can get out of the 256 MB limited C
heap considering that there are other things that get malloced too.<br>
<br>
So, the problems occur mostly on Solaris x86. My suggested fix tries
to address this by letting the task queues be mapped instead of
malloced on Solaris x86. Instead of inlining this logic in
taskqueue.cpp I added a more general class. The reason for this is
that I think we need to use the same logic in more places,
especially for G1, which is mallocing quite a lot.<br>
<br>
Since I think malloc on other platforms use mapped memory for large
malloc requests I think it is enough for this change to have effect
on Solaris. The other platforms probably have better heuristics than
I can come up with for which sizes should be mapped. On Sparc we
have the same limitation with malloc, but we have more memory
available for the C heap. This is why I have only enabled this for
Solaris x86.<br>
<br>
Also, I will be on vacation for a few days. Back in the office
Thrusday April 4. I'm happy for any feedback on this, but if I don't
respond until next week you know why :)<br>
<br>
Thanks,<br>
Bengt<br>
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