theora dev - Jul 2008 - GSoC - Theora multithread decoder

Hi all,

I apologize to not keep you up to date to what is going on with my project.
Portavales has worked in a desk behind me and when we go to take coffee we
talk about the project. Second I didn't know we have to discuss weekly, it
was my fault. I should have read the rules. Sorry.

At the first month, I studied the code and the Theora Beta implementation.
The code is completely different from Alpha and I have to be familiarized
with the code.
After that I started doing tests with OpenMP.

One first test was 40% faster, but unfortunately it did not decode the frame
correctly, three quarters was green.

I have one implementation decoding the Y, Cb and Cr planes in parallel. The
OpenMP implementation was about 5% faster. Not worthless, since it does not
require any great modifications.

I looked at Ralph's implementation and merged it to the current. The speed
up was about 10% but the code have to be modified in many places.

Extract parallelism from the current implementation is very difficult.
Coarse grain functions are the best functions to be parallelize to become
the overhead worthwhile, but the current implementation has one, at most
two. The parts that I suggested in my initial plan are fine grain functions,
they spend a lot of cpu time but they are called too many times. The time
spent to create and synchronize threads is greater than the speed up gains.
We need functions that are called a few times and spend many cpu time. Also
data dependency should be the lowest as possible.

According to the model that i did (
http://lampiao.lsc.ic.unicamp.br/~piga/gsoc_2008/implementation.pdf<http://lampiao.lsc.ic.unicamp.br/%7Epiga/gsoc_2008/implementation.pdf>)
the decoding time should be reduced in 33%, but it was just 10% for pthread
an 5% for openMP.

I used a video with 1440x1080. The pthread implementation has 3 threads and
the OpenMP was executed with the environment variable OMP_NUM_THREADS=3. The
results are:

                   Real(s)         User(s)             System(s)
Speed up(%)
OpenMP      25.2             29.2                  1.8
4
PThread       23.8             28.3                  1.0
  10
Current        26.2             26.0
0.3                       0

I used an Intel(R) Core(TM)2 Quad CPU with 2.4GHz and RAM of 4GB. The video
has 85 seconds.
These two implementations decode the Y, Cb and Cr planes in parallel, that
is why I am using OMP_NUM_THREADS=3 and the upper bound gain is 33%, that
is, let To be the time spent in decoding a video with the current
implementation. Let T1 be a video decoded with the parallel implementation.
T1 should be at most 0.66To.

I will use the pthread implementation to try a pipelined version and see if
we obtain more gains.
These version will run the functions (c_dec_dc_unpredict_mcu_plane +
oc_dec_frags_recon_mcu_plane) and
(oc_state_loop_filter_frag_rows + oc_state_borders_fill_rows) in parallel.
The upper bound for the gain is 60%, that is, let T2 be a video decoded with
the pipelined implementation. T2 should be at most 0.4To.

Here is the branch for the OpenMP implementation:
http://svn.xiph.org/branches/theora_multithread_decode_omp/
Here is the branch for the PThread implementation:
http://svn.xiph.org/branches/theora_multithread_decode_pthread/





Again, sorry about the long time without any feedback.

-- 
Leonardo de Paula Rosa Piga
Undergraduate Computer Engineering Student
LSC - IC - UNICAMP
http://lampiao.lsc.ic.unicamp.br/~piga<http://lampiao.lsc.ic.unicamp.br/%7Epiga>
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This week I will work with the pipeline and by the end of this week I will
send a report.


On Sun, Jul 6, 2008 at 9:39 PM, Leonardo de Paula Rosa Piga <
lpiga at terra.com.br> wrote:
> Hi all,
>
> I apologize to not keep you up to date to what is going on with my project.
> Portavales has worked in a desk behind me and when we go to take coffee we
> talk about the project. Second I didn't know we have to discuss weekly,
it
> was my fault. I should have read the rules. Sorry.
>
> At the first month, I studied the code and the Theora Beta implementation.
> The code is completely different from Alpha and I have to be familiarized
> with the code.
> After that I started doing tests with OpenMP.
>
> One first test was 40% faster, but unfortunately it did not decode the
> frame correctly, three quarters was green.
>
> I have one implementation decoding the Y, Cb and Cr planes in parallel. The
> OpenMP implementation was about 5% faster. Not worthless, since it does not
> require any great modifications.
>
> I looked at Ralph's implementation and merged it to the current. The
speed
> up was about 10% but the code have to be modified in many places.
>
> Extract parallelism from the current implementation is very difficult.
> Coarse grain functions are the best functions to be parallelize to become
> the overhead worthwhile, but the current implementation has one, at most
> two. The parts that I suggested in my initial plan are fine grain
functions,
> they spend a lot of cpu time but they are called too many times. The time
> spent to create and synchronize threads is greater than the speed up gains.
> We need functions that are called a few times and spend many cpu time. Also
> data dependency should be the lowest as possible.
>
> According to the model that i did (
>
http://lampiao.lsc.ic.unicamp.br/~piga/gsoc_2008/implementation.pdf<http://lampiao.lsc.ic.unicamp.br/%7Epiga/gsoc_2008/implementation.pdf>)
> the decoding time should be reduced in 33%, but it was just 10% for pthread
> an 5% for openMP.
>
> I used a video with 1440x1080. The pthread implementation has 3 threads and
> the OpenMP was executed with the environment variable OMP_NUM_THREADS=3.
The
> results are:
>
>                    Real(s)         User(s)
> System(s)             Speed up(%)
> OpenMP      25.2             29.2                  1.8
>   4
> PThread       23.8             28.3                  1.0
>     10
> Current        26.2             26.0
> 0.3                       0
>
> I used an Intel(R) Core(TM)2 Quad CPU with 2.4GHz and RAM of 4GB. The video
> has 85 seconds.
> These two implementations decode the Y, Cb and Cr planes in parallel, that
> is why I am using OMP_NUM_THREADS=3 and the upper bound gain is 33%, that
> is, let To be the time spent in decoding a video with the current
> implementation. Let T1 be a video decoded with the parallel implementation.
> T1 should be at most 0.66To.
>
> I will use the pthread implementation to try a pipelined version and see if
> we obtain more gains.
> These version will run the functions (c_dec_dc_unpredict_mcu_plane +
> oc_dec_frags_recon_mcu_plane) and
> (oc_state_loop_filter_frag_rows + oc_state_borders_fill_rows) in parallel.
> The upper bound for the gain is 60%, that is, let T2 be a video decoded
with
> the pipelined implementation. T2 should be at most 0.4To.
>
> Here is the branch for the OpenMP implementation:
> http://svn.xiph.org/branches/theora_multithread_decode_omp/
> Here is the branch for the PThread implementation:
> http://svn.xiph.org/branches/theora_multithread_decode_pthread/
>
>
>
>
>
> Again, sorry about the long time without any feedback.
>
> --
> Leonardo de Paula Rosa Piga
> Undergraduate Computer Engineering Student
> LSC - IC - UNICAMP
>
http://lampiao.lsc.ic.unicamp.br/~piga<http://lampiao.lsc.ic.unicamp.br/%7Epiga>



-- 
Leonardo de Paula Rosa Piga
Undergraduate Computer Engineering Student
LSC - IC - UNICAMP
http://lampiao.lsc.ic.unicamp.br/~piga
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Leonardo de Paula Rosa Piga wrote:
> Coarse grain functions are the best functions to be parallelize to
> become the overhead worthwhile, but the current implementation has one,
> at most two. The parts that I suggested in my initial plan are fine
The reason the current decoder does this is cache coherency. The idea is
that only a few (16 to 32) rows need to be kept in L1/L2 cache between
each stage of the pipeline, which is a big reason the current decoder is
as fast as it is on high resolution content.

It's possible break this pipeline back up into separate stages that
operate on the entire frame at once (e.g., just make the MCU size the
height of the frame). You lose cache coherency, but get coarse-grained
parallelism. Only testing will determine which is the better strategy.
> the decoding time should be reduced in 33%, but it was just 10% for
> pthread an 5% for openMP.
The chroma coefficients are usually quantized much more coarsely, so
they very likely don't account for a full 33% of the decode time even on
a uniprocessor. Fewer coded blocks and fewer tokens to unpack in the
blocks that are coded means fewer and smaller iDCTs, fewer invocations
of the loop filter, etc.

It's sad that OpenMP didn't do better... I was hoping with the option
available to them to do platform-specific tricks, they could cut down on
the overhead of pthreads, but I guess that stuff's just not
"there" yet.
> These version will run the functions (c_dec_dc_unpredict_mcu_plane +
> oc_dec_frags_recon_mcu_plane) and
> (oc_state_loop_filter_frag_rows + oc_state_borders_fill_rows) in
> parallel. The upper bound for the gain is 60%, that is, let T2 be a
> video decoded with the pipelined implementation. T2 should be at most
0.4To.
I think you mean "at least". Let us know what your test results look
like (good or bad)! Keep in mind that, if possible, the same thread that
does oc_dec_dc_unpredict_mcu_plane+oc_dec_frags_recon_mcu_plane on a set
of blocks should also be the one to do
oc_state_loop_filter_frag_rows+oc_state_borders_fill_rows on the same
set of blocks (and hopefully the scheduler doesn't muck things up by
moving the thread to a different physical CPU inbetween).