[armedbear-devel] Profiling ABCL exception handling

[Erik Huelsmann]

Hi Joel,

On Tue, Nov 9, 2010 at 11:00 AM, Erik Huelsmann <ehuels at gmail.com> wrote:
> Hi Joel,
>
>>> Running an exception profile on Maxima running its test suite shows
>>> lots of non-local transfer of control exceptions: Return, Go and
>>> Throw: during a run of ca 10 minutes, I'm finding 5.5 million
>>> exceptions being thrown which - as MissionControl calculated for me -
>>> should mean an average of little over 12k/s. I guess that's kind of
>>> expected with our strategy for non-local transfers of control.
>>
>> To put this into context we (developers at JRockit) expect ~1000
>> exceptions / core / sec from a "typical" Weblogic app. We define this
>> as a bit much but not terribly excessive.
>>
>> My guess is that between the horrible support for fixnums (and the
>> really slow bignums) in Java and the order of magnitude higher
>> exception count this will dominate your performance.

Trying to find out how much it matters, I changed ABCL's compiler
according to my ideas in the mail I stated earlier today. It seems
MissionControl indeed measures the number of 'athrow' opcodes
executed. Reducing the number of try/catch blocks (and hence the
number of rethrows) by 60% [measured during the first 3 minutes of
Maxima's test suite],  I was able to reduce the total Maxima test
suite evaluation time by a whopping 2% :-)

One of the explanations I can come up with is that our ControlTransfer
exceptions don't snapshot the stack: we've disabled stack snapshotting
on them because we found the impact of stack snapshots to be huge. In
our case, ControlTransfer is just an exceptional situation, not an
error, so, we're 99,999999999% of the time not interested in the
stacktrace it comes with.

I do think however that code resulting from the design that I wrote
down is much cleaner in terms of number of exception blocks etc. So, I
might commit it anyway. I'll still need to clean up the interpreter
part of things though.


Do you have some further ideas for profiling ABCL, possibly with Maxima?


Bye,

Erik.