Why is ParallelDo slower than Do?
$begingroup$
I have problems to write parallel code in mathematica.
Why is
candidates = {};
SetSharedVariable[candidates];
Do[
ParallelDo[
eq = RandomReal + RandomReal;
AppendTo[candidates, eq]
, {j, 1, 1000}]
, {i, 1, 10}]
slower than the non parallel version
candidates = {};
Do[
Do[
eq = RandomReal + RandomReal;
AppendTo[candidates, eq]
, {j, 1, 1000}]
, {i, 1, 10}]
?
parallelization
edited yesterday
corey979
20.9k64282
asked yesterday
Matthias HellerMatthias Heller
283
New contributor
Matthias Heller is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
add a comment |
$begingroup$
I have problems to write parallel code in mathematica.
Why is
candidates = {};
SetSharedVariable[candidates];
Do[
ParallelDo[
eq = RandomReal + RandomReal;
AppendTo[candidates, eq]
, {j, 1, 1000}]
, {i, 1, 10}]
slower than the non parallel version
candidates = {};
Do[
Do[
eq = RandomReal + RandomReal;
AppendTo[candidates, eq]
, {j, 1, 1000}]
, {i, 1, 10}]
?
parallelization
edited yesterday
corey979
20.9k64282
asked yesterday
Matthias HellerMatthias Heller
283
New contributor
Matthias Heller is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
$begingroup$
I reverted your post to before the edit because it looks like a different question (like Henrik said in his comment). Note, however, that if you ask it in precisely such form it will be likely closed due to not enough info: you need to provide the minimal working example, not through some undefined functions into a piece of code that no one will be able to run and test.
$endgroup$
– corey979
yesterday
1
$begingroup$
See here mathematica.stackexchange.com/a/48296/12 I suggest you don't use SetSharedVariable until you get quite fluent in using the parallel tools. It effectively "unparallelizes" your code.
$endgroup$
– Szabolcs
yesterday
add a comment |
$begingroup$
I have problems to write parallel code in mathematica.
Why is
candidates = {};
SetSharedVariable[candidates];
Do[
ParallelDo[
eq = RandomReal + RandomReal;
AppendTo[candidates, eq]
, {j, 1, 1000}]
, {i, 1, 10}]
slower than the non parallel version
candidates = {};
Do[
Do[
eq = RandomReal + RandomReal;
AppendTo[candidates, eq]
, {j, 1, 1000}]
, {i, 1, 10}]
?
parallelization
edited yesterday
corey979
20.9k64282
asked yesterday
Matthias HellerMatthias Heller
283
New contributor
Matthias Heller is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
I have problems to write parallel code in mathematica.
Why is
candidates = {};
SetSharedVariable[candidates];
Do[
ParallelDo[
eq = RandomReal + RandomReal;
AppendTo[candidates, eq]
, {j, 1, 1000}]
, {i, 1, 10}]
slower than the non parallel version
candidates = {};
Do[
Do[
eq = RandomReal + RandomReal;
AppendTo[candidates, eq]
, {j, 1, 1000}]
, {i, 1, 10}]
?
parallelization
parallelization
edited yesterday
corey979
20.9k64282
asked yesterday
Matthias HellerMatthias Heller
283
New contributor
Matthias Heller is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited yesterday
corey979
20.9k64282
asked yesterday
Matthias HellerMatthias Heller
283
New contributor
Matthias Heller is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited yesterday
corey979
20.9k64282
edited yesterday
corey979
20.9k64282
edited yesterday
corey979
20.9k64282
20.9k64282
asked yesterday
Matthias HellerMatthias Heller
283
New contributor
Matthias Heller is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked yesterday
Matthias HellerMatthias Heller
283
asked yesterday
Matthias HellerMatthias Heller
283
283
New contributor
Matthias Heller is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
Matthias Heller is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
Matthias Heller is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$begingroup$
I reverted your post to before the edit because it looks like a different question (like Henrik said in his comment). Note, however, that if you ask it in precisely such form it will be likely closed due to not enough info: you need to provide the minimal working example, not through some undefined functions into a piece of code that no one will be able to run and test.
$endgroup$
– corey979
yesterday
1
$begingroup$
See here mathematica.stackexchange.com/a/48296/12 I suggest you don't use SetSharedVariable until you get quite fluent in using the parallel tools. It effectively "unparallelizes" your code.
$endgroup$
– Szabolcs
yesterday
add a comment |
$begingroup$
I reverted your post to before the edit because it looks like a different question (like Henrik said in his comment). Note, however, that if you ask it in precisely such form it will be likely closed due to not enough info: you need to provide the minimal working example, not through some undefined functions into a piece of code that no one will be able to run and test.
$endgroup$
– corey979
yesterday
1
$begingroup$
See here mathematica.stackexchange.com/a/48296/12 I suggest you don't use SetSharedVariable until you get quite fluent in using the parallel tools. It effectively "unparallelizes" your code.
$endgroup$
– Szabolcs
yesterday
$begingroup$
I reverted your post to before the edit because it looks like a different question (like Henrik said in his comment). Note, however, that if you ask it in precisely such form it will be likely closed due to not enough info: you need to provide the minimal working example, not through some undefined functions into a piece of code that no one will be able to run and test.
$endgroup$
– corey979
yesterday
$begingroup$
I reverted your post to before the edit because it looks like a different question (like Henrik said in his comment). Note, however, that if you ask it in precisely such form it will be likely closed due to not enough info: you need to provide the minimal working example, not through some undefined functions into a piece of code that no one will be able to run and test.
$endgroup$
– corey979
yesterday
1
1
$begingroup$
See here mathematica.stackexchange.com/a/48296/12 I suggest you don't use SetSharedVariable until you get quite fluent in using the parallel tools. It effectively "unparallelizes" your code.
$endgroup$
– Szabolcs
yesterday
$begingroup$
See here mathematica.stackexchange.com/a/48296/12 I suggest you don't use SetSharedVariable until you get quite fluent in using the parallel tools. It effectively "unparallelizes" your code.
$endgroup$
– Szabolcs
yesterday
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
Because managing write access to shared memory is expensive: Subprocesses have to wait until they are granted write access (because another process uses that ressource).
Moreover, it is in general more efficient to use Parallel only upon the most outer loop construct.
By the way: Using Append and AppendTo are the worst methods to build a list, because they involve a copy of the full list each time another element is appended. Instead of complexity $O(n)$ for a list of $n$ elements, you get an implementation of complexity $O(n^2)$. Better use Table or, if you don't know how long the list is about to get, use Sow and Reap. Internal`Bag is a further option, and it is even compilable.
answered yesterday
Henrik SchumacherHenrik Schumacher
60k582168
$endgroup$
$begingroup$
Thanks, that actually helped a lot. I just dont understand how to use Sow and Reap to avoid Append To be more specific: instead of ParallelDo I use now ParallelTable: eq = ParallelTable[ FNumeric[ SetPrecision[N[monlistnumeric[[i]] + monlistnumeric[[j]], 20], 10]] , {j, jj}]; FNumeric is a function, that returns either 0 or a value I want to store. I then do eq = DeleteCases[eq, 0]; candidates = Join[candidates, eq]; Is there a more efficient way to do this?
$endgroup$
– Matthias Heller
yesterday
2
$begingroup$
@MatthiasHeller, you're welcome. How is this new code related to your post? You should consider a new post with your real problem and all relevant data. I may have a look. In general, depending on the details, there are various ways to perform the computation efficiently; these way might not useParallelat all, but ratherCompiled code.
$endgroup$
– Henrik Schumacher
yesterday
add a comment |
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1 Answer
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active
oldest
votes
1 Answer
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active
oldest
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active
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votes
$begingroup$
Because managing write access to shared memory is expensive: Subprocesses have to wait until they are granted write access (because another process uses that ressource).
Moreover, it is in general more efficient to use Parallel only upon the most outer loop construct.
By the way: Using Append and AppendTo are the worst methods to build a list, because they involve a copy of the full list each time another element is appended. Instead of complexity $O(n)$ for a list of $n$ elements, you get an implementation of complexity $O(n^2)$. Better use Table or, if you don't know how long the list is about to get, use Sow and Reap. Internal`Bag is a further option, and it is even compilable.
answered yesterday
Henrik SchumacherHenrik Schumacher
60k582168
$endgroup$
$begingroup$
Thanks, that actually helped a lot. I just dont understand how to use Sow and Reap to avoid Append To be more specific: instead of ParallelDo I use now ParallelTable: eq = ParallelTable[ FNumeric[ SetPrecision[N[monlistnumeric[[i]] + monlistnumeric[[j]], 20], 10]] , {j, jj}]; FNumeric is a function, that returns either 0 or a value I want to store. I then do eq = DeleteCases[eq, 0]; candidates = Join[candidates, eq]; Is there a more efficient way to do this?
$endgroup$
– Matthias Heller
yesterday
2
$begingroup$
@MatthiasHeller, you're welcome. How is this new code related to your post? You should consider a new post with your real problem and all relevant data. I may have a look. In general, depending on the details, there are various ways to perform the computation efficiently; these way might not useParallelat all, but ratherCompiled code.
$endgroup$
– Henrik Schumacher
yesterday
add a comment |
$begingroup$
Because managing write access to shared memory is expensive: Subprocesses have to wait until they are granted write access (because another process uses that ressource).
Moreover, it is in general more efficient to use Parallel only upon the most outer loop construct.
By the way: Using Append and AppendTo are the worst methods to build a list, because they involve a copy of the full list each time another element is appended. Instead of complexity $O(n)$ for a list of $n$ elements, you get an implementation of complexity $O(n^2)$. Better use Table or, if you don't know how long the list is about to get, use Sow and Reap. Internal`Bag is a further option, and it is even compilable.
answered yesterday
Henrik SchumacherHenrik Schumacher
60k582168
$endgroup$
$begingroup$
Thanks, that actually helped a lot. I just dont understand how to use Sow and Reap to avoid Append To be more specific: instead of ParallelDo I use now ParallelTable: eq = ParallelTable[ FNumeric[ SetPrecision[N[monlistnumeric[[i]] + monlistnumeric[[j]], 20], 10]] , {j, jj}]; FNumeric is a function, that returns either 0 or a value I want to store. I then do eq = DeleteCases[eq, 0]; candidates = Join[candidates, eq]; Is there a more efficient way to do this?
$endgroup$
– Matthias Heller
yesterday
2
$begingroup$
@MatthiasHeller, you're welcome. How is this new code related to your post? You should consider a new post with your real problem and all relevant data. I may have a look. In general, depending on the details, there are various ways to perform the computation efficiently; these way might not useParallelat all, but ratherCompiled code.
$endgroup$
– Henrik Schumacher
yesterday
add a comment |
$begingroup$
Because managing write access to shared memory is expensive: Subprocesses have to wait until they are granted write access (because another process uses that ressource).
Moreover, it is in general more efficient to use Parallel only upon the most outer loop construct.
By the way: Using Append and AppendTo are the worst methods to build a list, because they involve a copy of the full list each time another element is appended. Instead of complexity $O(n)$ for a list of $n$ elements, you get an implementation of complexity $O(n^2)$. Better use Table or, if you don't know how long the list is about to get, use Sow and Reap. Internal`Bag is a further option, and it is even compilable.
answered yesterday
Henrik SchumacherHenrik Schumacher
60k582168
$endgroup$
Because managing write access to shared memory is expensive: Subprocesses have to wait until they are granted write access (because another process uses that ressource).
Moreover, it is in general more efficient to use Parallel only upon the most outer loop construct.
By the way: Using Append and AppendTo are the worst methods to build a list, because they involve a copy of the full list each time another element is appended. Instead of complexity $O(n)$ for a list of $n$ elements, you get an implementation of complexity $O(n^2)$. Better use Table or, if you don't know how long the list is about to get, use Sow and Reap. Internal`Bag is a further option, and it is even compilable.
answered yesterday
Henrik SchumacherHenrik Schumacher
60k582168
edited yesterday
answered yesterday
Henrik SchumacherHenrik Schumacher
60k582168
answered yesterday
Henrik SchumacherHenrik Schumacher
60k582168
answered yesterday
Henrik SchumacherHenrik Schumacher
60k582168
60k582168
$begingroup$
Thanks, that actually helped a lot. I just dont understand how to use Sow and Reap to avoid Append To be more specific: instead of ParallelDo I use now ParallelTable: eq = ParallelTable[ FNumeric[ SetPrecision[N[monlistnumeric[[i]] + monlistnumeric[[j]], 20], 10]] , {j, jj}]; FNumeric is a function, that returns either 0 or a value I want to store. I then do eq = DeleteCases[eq, 0]; candidates = Join[candidates, eq]; Is there a more efficient way to do this?
$endgroup$
– Matthias Heller
yesterday
2
$begingroup$
@MatthiasHeller, you're welcome. How is this new code related to your post? You should consider a new post with your real problem and all relevant data. I may have a look. In general, depending on the details, there are various ways to perform the computation efficiently; these way might not useParallelat all, but ratherCompiled code.
$endgroup$
– Henrik Schumacher
yesterday
add a comment |
$begingroup$
Thanks, that actually helped a lot. I just dont understand how to use Sow and Reap to avoid Append To be more specific: instead of ParallelDo I use now ParallelTable: eq = ParallelTable[ FNumeric[ SetPrecision[N[monlistnumeric[[i]] + monlistnumeric[[j]], 20], 10]] , {j, jj}]; FNumeric is a function, that returns either 0 or a value I want to store. I then do eq = DeleteCases[eq, 0]; candidates = Join[candidates, eq]; Is there a more efficient way to do this?
$endgroup$
– Matthias Heller
yesterday
2
$begingroup$
@MatthiasHeller, you're welcome. How is this new code related to your post? You should consider a new post with your real problem and all relevant data. I may have a look. In general, depending on the details, there are various ways to perform the computation efficiently; these way might not useParallelat all, but ratherCompiled code.
$endgroup$
– Henrik Schumacher
yesterday
$begingroup$
Thanks, that actually helped a lot. I just dont understand how to use Sow and Reap to avoid Append To be more specific: instead of ParallelDo I use now ParallelTable: eq = ParallelTable[ FNumeric[ SetPrecision[N[monlistnumeric[[i]] + monlistnumeric[[j]], 20], 10]] , {j, jj}]; FNumeric is a function, that returns either 0 or a value I want to store. I then do eq = DeleteCases[eq, 0]; candidates = Join[candidates, eq]; Is there a more efficient way to do this?
$endgroup$
– Matthias Heller
yesterday
$begingroup$
Thanks, that actually helped a lot. I just dont understand how to use Sow and Reap to avoid Append To be more specific: instead of ParallelDo I use now ParallelTable: eq = ParallelTable[ FNumeric[ SetPrecision[N[monlistnumeric[[i]] + monlistnumeric[[j]], 20], 10]] , {j, jj}]; FNumeric is a function, that returns either 0 or a value I want to store. I then do eq = DeleteCases[eq, 0]; candidates = Join[candidates, eq]; Is there a more efficient way to do this?
$endgroup$
– Matthias Heller
yesterday
2
2
$begingroup$
@MatthiasHeller, you're welcome. How is this new code related to your post? You should consider a new post with your real problem and all relevant data. I may have a look. In general, depending on the details, there are various ways to perform the computation efficiently; these way might not use
Parallel at all, but rather Compiled code.$endgroup$
– Henrik Schumacher
yesterday
$begingroup$
@MatthiasHeller, you're welcome. How is this new code related to your post? You should consider a new post with your real problem and all relevant data. I may have a look. In general, depending on the details, there are various ways to perform the computation efficiently; these way might not use
Parallel at all, but rather Compiled code.$endgroup$
– Henrik Schumacher
yesterday
add a comment |
Matthias Heller is a new contributor. Be nice, and check out our Code of Conduct.
Matthias Heller is a new contributor. Be nice, and check out our Code of Conduct.
Matthias Heller is a new contributor. Be nice, and check out our Code of Conduct.
Matthias Heller is a new contributor. Be nice, and check out our Code of Conduct.
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$begingroup$
I reverted your post to before the edit because it looks like a different question (like Henrik said in his comment). Note, however, that if you ask it in precisely such form it will be likely closed due to not enough info: you need to provide the minimal working example, not through some undefined functions into a piece of code that no one will be able to run and test.
$endgroup$
– corey979
yesterday
1
$begingroup$
See here mathematica.stackexchange.com/a/48296/12 I suggest you don't use SetSharedVariable until you get quite fluent in using the parallel tools. It effectively "unparallelizes" your code.
$endgroup$
– Szabolcs
yesterday