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From "Geir Magnusson Jr." <g...@pobox.com>
Subject Re: [drlvm] stress.Mix / MegaSpawn threading bug
Date Wed, 10 Jan 2007 13:11:46 GMT

On Jan 9, 2007, at 1:14 PM, Rana Dasgupta wrote:

> On 1/9/07, Weldon Washburn <weldonwjw@gmail.com> wrote:
>> On 1/9/07, Gregory Shimansky <gshimansky@gmail.com> wrote:
>> >> I've tried to analyze MegaSpawn test on windows and here's what  
>> I found
>> >> out.
>> >>
>> >> OOME is thrown because process virtual size easily gets up to  
>> 2Gb. This
>> >> happens at about ~1.5k simultaneously running threads. I think it
>> >> happens because all of virtual process memory is mapped for thread
>> stacks.
>> >>
>> >Good job!  I got the same sort of hunch when I looked at the  
>> source code
>> did
>> >not have enough time to pin down specifics.  The only guidance I
>> >found regarding what happens when too many threads are spawned is  
>> the
>> >following in the java.lang.Thread reference manual, "...specifying a
>> lower
>> >[stacksize] value may allow a greater number of threads to exist
>> >concurrently without throwing an OutOfMemoryError (or other internal
>> >error)."
>> >I think what the above implies is that it is OK for the JVM to  
>> error and
>> >exit if the app tries to create too many threads.  If this is the  
>> case,
>> it
>> >sort of looks like we need to clean up the handling of malloc()  
>> errors so
>> >that the JVM can exit gracefully.
> I am not sure that we need to do something about this. The default  
> initial
> stack size on Windows is 1M,

Yikes!  There's our problem on windows...

> and that is the recommended init size for real
> applications. The fact that our threads start with a larger intial  
> stack
> mapped( default ) than RI is a design issue, it is not a bug. We  
> could start
> with 2K and create many more threads!

That's right.  The fact that the VM crashes and burns is the bug, and  
a serious one, IMO.

> Exactly as Gregory points out,
> ultimately we will hit virtual memory limits and fail. The reason  
> the RI
> seems to fail less is that the test ends before running out of virtual
> memory.On my 32 bit RHEL Linux box, RI fails almost every time with
> MegaSpawn, with an identical OOME error message and stack dump.
> We can catch the exception in the test and print a message. But I  
> am not
> very sure what purpose that would serve. A resource exhaustion  
> exception is
> a fatal exception and the process is hosed,

No, it's not.

> no real app would be able to do
> anything more at this point.

That's not true.

> We should not use this test ( which is not a
> real app ) as guidance to tune the initial stack size. My  
> suggestion is to
> lower the test duration so that we can create about a 1000( or  
> whatever
> magic number ) threads at least. That is the stress condition we  
> should test
> for.

The big thing for me is ensuring that we can drive the VM to the  
limit, and it maintains internal integrity, so applications that are  
designed to gracefully deal with resource exhaustion can do so w/  
confidence that the VM isn't about to crumble out from underneath them.


> a
> Thanks,
> Rana

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