Re: [boost] [serialization] fast array serialization (10x speedup)

From: Robert Ramey (ramey_at_[hidden])
Date: 2005-11-25 17:35:42

Matthias Troyer wrote:

>>> Suggestions
>>> ===========
>>>
>>> a) Do more work in finding the speed bottlenecks.  Run
>>> a profiler. Make a buffer based non-stream based archive
>>> and re-run your tests.
>>
>> I have attached a benchmark for such an archive class and ran
>> benchmarks for std::vector<char> serialization. Here are the numbers
>> (using gcc-4 on a Powerbook G4):

I've take a look at your benchmark.cpp.

First of all its very nice and simple and shows an understanding how
the primitive i/o is isolated from the archives that use it.

Its a step in the right direction.  But I see some problems.  The usage
of std::vector<char> isn't what I would expect for an output buffer.
You arn't using this in your own archives are you?

Here are my timing results on my windoz XP system with a
2.4 gHz pentiem.

With your original program I get

for value_type set char

Time using serialization library: 9.454
Size is 100000004
Time using direct calls to save in a loop: 8.844
Size is 100000000
Time using direct call to save_array: 0.266
Size is 100000000
for value type set to double

Time using serialization library: 1.281
Size is 100000004
Time using direct calls to save in a loop: 1.218
Size is 100000000
Time using direct call to save_array: 0.266
Size is 100000000

I modified the to use a simple buffer output closer to what I would
expect to use if I were going to make a primitive buffer output.
BTW - that would be a very nice addition.  This would be
much faster than using strstream as is being use now.

Here I the results with the program modified in this way

For value type set to char I get

Time using serialization library: 0.797
Size is 100000004
Time using direct calls to save in a loop: 0.297 (1)
Size is 100000000
Time using direct call to save_array: 0.203
Size is 100000000

and for value_type set to double I get

Time using serialization library: 0.109 (3)
Size is 100000004
Time using direct calls to save in a loop: 0.078 (2)
Size is 100000000
Time using direct call to save_array: 0.25
Size is 100000000

a) the usage of save_array does not have a huge
effect on performance.  It IS measureable.  It seems
that it saves about 1/3 the time over using a loop
of saves in the best case. (1)

b) In the worst case, its even slower than a loop of saves!!! (2)
and even slower than the raw serialization system (3)

c) the overhead of the serialization library isn't too
bad.  It does show up when doing 100M characters
one by one, but generally it doesn't seem to be a big
issuues.

In my view, it does support my contention that
implementing save_array - regardless of how it is
in fact implemented - represents a premature optimization.
I suspect that the net benefit in the kind of scenario you
envision using it will be very small.

Obviously, this test raises more questions than it
answers and I think it should be persued further.
Another thing I would like to see is a version
of the test applied to C++ arrays.  My interest
is to isolate bottlenecks in the serialization
library from those in the stl libraries.

Robert Ramey 

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