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Interesting discovery...

Austin Ziegler

6/1/2005 1:41:00 AM

I had a user report a slowdown in PDF::Writer. I'm pretty certain I
know where the problem is, but I found something very interesting.
About 3% of the profiled performance of the manual generation (with
Shugo Maeda's new profiler) -- and near 20,000,000 (yes twenty
MILLION) calls -- are calls to Kernel.respond_to?.

A little digging later, and it looks like 98% of these calls -- or
more -- are in Marshal.dump. That's ... amazing.

-austin
--
Austin Ziegler * halostatue@gmail.com
* Alternate: austin@halostatue.ca
10 Answers

Ara.T.Howard

6/1/2005 3:03:00 AM

0

Austin Ziegler

6/1/2005 12:09:00 PM

0

On 5/31/05, Ara.T.Howard <Ara.T.Howard@noaa.gov> wrote:
> On Wed, 1 Jun 2005, Austin Ziegler wrote:
>> I had a user report a slowdown in PDF::Writer. I'm pretty certain
>> I know where the problem is, but I found something very
>> interesting. About 3% of the profiled performance of the manual
>> generation (with Shugo Maeda's new profiler) -- and near
>> 20,000,000 (yes twenty MILLION) calls -- are calls to
>> Kernel.respond_to?.
>>
>> A little digging later, and it looks like 98% of these calls --
>> or more -- are in Marshal.dump. That's ... amazing.
> wow. can you confirm with somthing like

I've done quite a bit of confirmation. Much of this has to do with
the unique nature of the object graph in PDF::Writer documents --
there's quite a bit of depth to the object graph.

I was originally investigating Transaction::Simple as the culprit of
my number of calls, but that didn't mesh with what I *did* find. My
final test was to do:

def Kernel.respond_to?(*a)
puts caller[0]
old_respond_to(*a)
end

After a few moments, I literally had tens of *thousands* of calls
indicating that the caller was Marshal.dump.

My guess -- and I haven't looked at the Marshal.dump code to confirm
-- is that what's beeing looked for is a custom dumper, and that
search is faster than assuming the dumper is there (because in most
cases, it won't be, I don't think).

-austin
--
Austin Ziegler * halostatue@gmail.com
* Alternate: austin@halostatue.ca

Ara.T.Howard

6/2/2005 2:28:00 AM

0

Joel VanderWerf

6/2/2005 7:44:00 PM

0

Ara.T.Howard@noaa.gov wrote:

> Benchmark::bm do |b|
> %w(rescue_test respond_to_test).map{|m| b.report(m){c.send m,n}}
> end
>
>
> harp:~ > ruby a.rb
> user system total real
> rescue_test 2.940000 0.320000 3.260000 ( 3.264758)
> respond_to_test 0.160000 0.050000 0.210000 ( 0.208268)
>
>
> looks like respond_to? is indeed faster.

But if the method *is* defined, rescue is faster:

class C
def a; end # <== added
def c; end
...


user system total real
rescue_test 0.070000 0.000000 0.070000 ( 0.072631)
respond_to_test 0.130000 0.010000 0.140000 ( 0.140980)


So, if some obejcts have the methods and some dont, the best choice
depends on the balance between the two branches (defined vs. undefined).

Ara.T.Howard

6/2/2005 8:06:00 PM

0

Jim Freeze

6/2/2005 9:09:00 PM

0

* Joel VanderWerf <vjoel@path.berkeley.edu> [2005-06-03 04:43:52 +0900]:

> Ara.T.Howard@noaa.gov wrote:
> > rescue_test 2.940000 0.320000 3.260000 ( 3.264758)
> > respond_to_test 0.160000 0.050000 0.210000 ( 0.208268)
> >
> >
> >looks like respond_to? is indeed faster.
>
> But if the method *is* defined, rescue is faster:

I'm not sure I understand. If the method exists, what is the
need for rescue to be called?

Are the options being tested these:

# with respond_to?
if x.respond_to(:meth) x.meth # or x.call(:meth)
# or with rescue
begin
x.meth
rescue
# no meth

So, if meth exists, the only overhead should be the 'if x.respond_to'.
If the method does nothing, then a check of this type may appear expensive,
but if the method does something, then the 'if' should not be too much
of a burden.
I suppose that this could be a problem in a tight loop. Under such
conditions, it may be better to just rescue.


--
Jim Freeze
Theory and practice are the same, in theory. -- Ryan Davis

Joel VanderWerf

6/2/2005 9:44:00 PM

0

Jim Freeze wrote:
> * Joel VanderWerf <vjoel@path.berkeley.edu> [2005-06-03 04:43:52 +0900]:
>
>
>>Ara.T.Howard@noaa.gov wrote:
>>
>>> rescue_test 2.940000 0.320000 3.260000 ( 3.264758)
>>> respond_to_test 0.160000 0.050000 0.210000 ( 0.208268)
>>>
>>>
>>>looks like respond_to? is indeed faster.
>>
>>But if the method *is* defined, rescue is faster:
>
>
> I'm not sure I understand. If the method exists, what is the
> need for rescue to be called?

I was unclear: I was hypothesizing a different test, in which a certain
proportion of the objects had the method and the rest did not. It might
be interesting to determine what ratio is the break-even point...

Jim Weirich

6/2/2005 9:46:00 PM

0


Ara.T.Howard@noaa.gov said:
> On Fri, 3 Jun 2005, Joel VanderWerf wrote:
>> But if the method *is* defined, rescue is faster:
>
> very true - i hadn't thought of that...

Now, if we could only find out if the method is defined, we could choose
the most efficient way of asking if a method is defined.

Or something like that.

--
-- Jim Weirich jim@weirichhouse.org http://onest...
-----------------------------------------------------------------
"Beware of bugs in the above code; I have only proved it correct,
not tried it." -- Donald Knuth (in a memo to Peter van Emde Boas)

Jim Freeze

6/2/2005 10:31:00 PM

0

* Joel VanderWerf <vjoel@path.berkeley.edu> [2005-06-03 06:43:40 +0900]:

> Jim Freeze wrote:
> >* Joel VanderWerf <vjoel@path.berkeley.edu> [2005-06-03 04:43:52 +0900]:

> >I'm not sure I understand. If the method exists, what is the
> >need for rescue to be called?
>
> I was unclear: I was hypothesizing a different test, in which a certain
> proportion of the objects had the method and the rest did not. It might
> be interesting to determine what ratio is the break-even point...

Yes, but doesn't that ratio depend upon the time taken in each method?

I think the rule of thumb is:

- If a missing method is a rare occurance, use rescue
- If a missing method is common and expected, use #respond_to
- If you don't know, then drop back ten yards and punt.

--
Jim Freeze
Theory and practice are the same, in theory. -- Ryan Davis

Eric Hodel

6/2/2005 10:59:00 PM

0

On 02 Jun 2005, at 12:43, Joel VanderWerf wrote:

> Ara.T.Howard@noaa.gov wrote:
>
>> Benchmark::bm do |b|
>> %w(rescue_test respond_to_test).map{|m| b.report(m){c.send m,n}}
>> end
>> harp:~ > ruby a.rb
>> user system total real
>> rescue_test 2.940000 0.320000 3.260000 ( 3.264758)
>> respond_to_test 0.160000 0.050000 0.210000 ( 0.208268)
>> looks like respond_to? is indeed faster.
>>
>
> But if the method *is* defined, rescue is faster:
>
> class C
> def a; end # <== added
> def c; end
>
> So, if some obejcts have the methods and some dont, the best choice
> depends on the balance between the two branches (defined vs.
> undefined).

class C
def a; Object.no_such_method; end
end

rescue NoMethodError is an inappropriate test for whether or not a
method exists. (This may or may not be important depending on what
what else you need to rescue.)

Being faster does not make it the correct solution.

--
Eric Hodel - drbrain@segment7.net - http://se...
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