llvm dev - Oct 2010 - [LLVMdev] Using multiple ExecutionEngines for better parallelism?

Hi,
I'm working on an application that compiles using LLVM from multiple 
threads. Currently, I'm doing this using one ExecutionEngine, but I've 
observed that this does not seem to scale well past a couple cores. I'd 
like to be able to get as much parallelism as possible in the 
compilation, and was wondering about using one ExecutionEngine per thread.
The only information I've found says this is not allowed, but the post 
is 2.5 years old, and it seems like threading in LLVM has come a long 
way since then. Has this changed, or is it still only possible to have 
one ExecutionEngine? Are there any other ways to increase the 
parallelism of LLVM compilation?

Thanks,
-George

Is is possible since LLVM 2.7 to create multiple ExecutionEngine in the same
process.

Olivier.


On Thu, Oct 7, 2010 at 6:41 PM, George Brewster <g at brewster.org> wrote:
> Hi,
> I'm working on an application that compiles using LLVM from multiple
> threads. Currently, I'm doing this using one ExecutionEngine, but
I've
> observed that this does not seem to scale well past a couple cores. I'd
> like to be able to get as much parallelism as possible in the
> compilation, and was wondering about using one ExecutionEngine per thread.
> The only information I've found says this is not allowed, but the post
> is 2.5 years old, and it seems like threading in LLVM has come a long
> way since then. Has this changed, or is it still only possible to have
> one ExecutionEngine? Are there any other ways to increase the
> parallelism of LLVM compilation?
>
> Thanks,
> -George
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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I set up a test case to try using multiple ExecutionEngines,  and it 
ends up coring, where a similar test case which uses only one 
ExecutionEngine works.

Looking at the failure, I found two threading issues that were causing 
crashes for me -- I've attached a patch for each one (against 
RELEASE_28). These patches are intended simply to demonstrate the 
problems, not to be the actual fix, as I know nothing of LLVM coding 
conventions or the larger plan for handling multi-threading, and there 
are certainly more clever ways to make these things thread safe.

The first, in Attributes.cpp, is a race condition relating to the static 
AttributesList. Access to the AttributesList is locked, but my theory 
for what was going on is that at line 125-126, when dropping a 
reference, we decrement the count, then check to see if it's zero, then 
delete "this" (if the refcount was zero). After we've checked that
the
count is zero, but before deleting "this", "this" is still
in the
AttributesLists set. So in the mean time, another thread comes along, 
finds "this" in AttributesList, and references it. But the first
thread
has already determined that the count *was* 0, and then comes back and 
deletes "this", even though it's now in use by the second thread.

The second, in DynamicLibrary.cpp, simply protects OpenedHandles with a 
mutex. Note that I couldn't use ManagedStatic with the mutex, as that 
would have introduced a circular library dependency (between Support and 
System).

I've also attached the test case that I was using, in case it will be 
useful in verifying different solutions to these problems.

-George



Olivier Meurant wrote:
> Is is possible since LLVM 2.7 to create multiple ExecutionEngine in 
> the same process.
>
> Olivier.
>
>
> On Thu, Oct 7, 2010 at 6:41 PM, George Brewster <g at brewster.org 
> <mailto:g at brewster.org>> wrote:
>
>     Hi,
>     I'm working on an application that compiles using LLVM from
multiple
>     threads. Currently, I'm doing this using one ExecutionEngine, but
I've
>     observed that this does not seem to scale well past a couple
>     cores. I'd
>     like to be able to get as much parallelism as possible in the
>     compilation, and was wondering about using one ExecutionEngine per
>     thread.
>     The only information I've found says this is not allowed, but the
post
>     is 2.5 years old, and it seems like threading in LLVM has come a long
>     way since then. Has this changed, or is it still only possible to have
>     one ExecutionEngine? Are there any other ways to increase the
>     parallelism of LLVM compilation?
>
>     Thanks,
>     -George
>     _______________________________________________
>     LLVM Developers mailing list
>     LLVMdev at cs.uiuc.edu <mailto:LLVMdev at cs.uiuc.edu>        
>     http://llvm.cs.uiuc.edu
>     http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
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