[lld] r232460 - [ELF] Use parallel_for_each for writing.
Shankar Easwaran
shankare at codeaurora.org
Tue Mar 17 14:57:57 PDT 2015
I tried to measure this again with 4 tries and got results, to make sure
just in case, and I see few results identical to what I measured before :-
_*Raw data below :-*_
LLD Try With Patch #1
4.16user 0.80system 0:03.06elapsed 162%CPU (0avgtext+0avgdata
7174160maxresident)k
LLD Try Without Patch #1
4.49user 0.92system 0:03.32elapsed 162%CPU (0avgtext+0avgdata
7179984maxresident)k
BFD Try #1
7.81user 0.68system 0:08.53elapsed 99%CPU (0avgtext+0avgdata
3230416maxresident)k
LLD Try With Patch #2
3.94user 0.86system 0:02.93elapsed 163%CPU (0avgtext+0avgdata
7175808maxresident)k
LLD Try Without Patch #2
4.12user 0.83system 0:03.22elapsed 154%CPU (0avgtext+0avgdata
7172704maxresident)k
BFD Try #2
7.78user 0.75system 0:08.57elapsed 99%CPU (0avgtext+0avgdata
3230416maxresident)k
LLD Try With Patch #3
4.36user 1.05system 0:03.08elapsed 175%CPU (0avgtext+0avgdata
7176320maxresident)k
LLD Try Without Patch #3
4.38user 0.90system 0:03.14elapsed 168%CPU (0avgtext+0avgdata
7175600maxresident)k
BFD Try #3
7.78user 0.64system 0:08.46elapsed 99%CPU (0avgtext+0avgdata
3230416maxresident)k
LLD Try With Patch #4
4.17user 0.72system 0:02.93elapsed 166%CPU (0avgtext+0avgdata
7175120maxresident)k
LLD Try Without Patch #4
4.20user 0.79system 0:03.08elapsed 161%CPU (0avgtext+0avgdata
7174864maxresident)k
BFD Try #4
7.77user 0.66system 0:08.46elapsed 99%CPU (0avgtext+0avgdata
3230416maxresident)k
_*Questions :-*_
As Rui mentions I dont know why the user time is more without the patch,
any methods to verify this ?
Could this be because of user threads instead of kernel threads ? *
*
Shankar Easwaran
On 3/17/2015 3:35 PM, Shankar Easwaran wrote:
> Yes, this is true. There were several logs of runs in the same file
> that I read into the commit and manually removing them resulted in two
> user lines.
>
> But the result for all reasons is true. I can re-measure the time
> taken though.
>
> Shankar Easwaran
>
> On 3/17/2015 2:30 PM, Rui Ueyama wrote:
>> On Mon, Mar 16, 2015 at 8:29 PM, Shankar Easwaran
>> <shankare at codeaurora.org>
>> wrote:
>>
>>> Author: shankare
>>> Date: Mon Mar 16 22:29:32 2015
>>> New Revision: 232460
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=232460&view=rev
>>> Log:
>>> [ELF] Use parallel_for_each for writing.
>>>
>>> This changes improves performance of lld, when self-hosting lld, when
>>> compared
>>> with the bfd linker. BFD linker on average takes 8 seconds in
>>> elapsed time.
>>> lld takes 3 seconds elapased time average. Without this change, lld
>>> takes
>>> ~5
>>> seconds average. The runtime comparisons were done on a release
>>> build and
>>> measured by running linking thrice.
>>>
>>> lld self-host without the change
>>> ----------------------------------
>>> real 0m3.196s
>>> user 0m4.580s
>>> sys 0m0.832s
>>>
>>> lld self-host with lld
>>> -----------------------
>>> user 0m3.024s
>>> user 0m3.252s
>>> sys 0m0.796s
>>>
>> The above results don't look real output of "time" command.
>>
>> If it's real, it's too good to be true, assuming the first line of the
>> second result is "real" instead of "user".
>>
>> "real" is wall clock time from process start to process exit. "user"
>> is CPU
>> time consumed by the process in user mode (if a process is
>> multi-threaded,
>> it can be larger than real).
>>
>> Your result shows significant improvement in user time. Which means you
>> have significantly reduced the amount of processing time to do the same
>> thing compared to before. However, because this change didn't change
>> algorithm, but just execute them in parallel, it couldn't happen.
>>
>> Something's not correct.
>>
>> I appreciate your effort to make LLD faster, but we need to be careful
>> about benchmark results. If we don't measure improvements accurately,
>> it's
>> easy to make an "optimization" that makes things slower.
>>
>> Another important thing is to disbelieve what you do when you optimize
>> something and measure its effect. It sometimes happen that I believe
>> something is going to improve performance 100% sure but it actually
>> wouldn't.
>>
>> time taken to build lld with bfd
>>> --------------------------------
>>> real 0m8.419s
>>> user 0m7.748s
>>> sys 0m0.632s
>>>
>>> Modified:
>>> lld/trunk/lib/ReaderWriter/ELF/OutputELFWriter.h
>>> lld/trunk/lib/ReaderWriter/ELF/SectionChunks.h
>>>
>>> Modified: lld/trunk/lib/ReaderWriter/ELF/OutputELFWriter.h
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/lld/trunk/lib/ReaderWriter/ELF/OutputELFWriter.h?rev=232460&r1=232459&r2=232460&view=diff
>>>
>>>
>>> ==============================================================================
>>>
>>> --- lld/trunk/lib/ReaderWriter/ELF/OutputELFWriter.h (original)
>>> +++ lld/trunk/lib/ReaderWriter/ELF/OutputELFWriter.h Mon Mar 16
>>> 22:29:32
>>> 2015
>>> @@ -586,8 +586,10 @@ std::error_code OutputELFWriter<ELFT>::w
>>> _elfHeader->write(this, _layout, *buffer);
>>> _programHeader->write(this, _layout, *buffer);
>>>
>>> - for (auto section : _layout.sections())
>>> - section->write(this, _layout, *buffer);
>>> + auto sections = _layout.sections();
>>> + parallel_for_each(
>>> + sections.begin(), sections.end(),
>>> + [&](Chunk<ELFT> *section) { section->write(this, _layout,
>>> *buffer);
>>> });
>>> writeTask.end();
>>>
>>> ScopedTask commitTask(getDefaultDomain(), "ELF Writer commit to
>>> disk");
>>>
>>> Modified: lld/trunk/lib/ReaderWriter/ELF/SectionChunks.h
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/lld/trunk/lib/ReaderWriter/ELF/SectionChunks.h?rev=232460&r1=232459&r2=232460&view=diff
>>>
>>>
>>> ==============================================================================
>>>
>>> --- lld/trunk/lib/ReaderWriter/ELF/SectionChunks.h (original)
>>> +++ lld/trunk/lib/ReaderWriter/ELF/SectionChunks.h Mon Mar 16
>>> 22:29:32 2015
>>> @@ -234,17 +234,17 @@ public:
>>> /// routine gets called after the linker fixes up the virtual
>>> address
>>> /// of the section
>>> virtual void assignVirtualAddress(uint64_t addr) override {
>>> - for (auto &ai : _atoms) {
>>> + parallel_for_each(_atoms.begin(), _atoms.end(), [&](AtomLayout
>>> *ai) {
>>> ai->_virtualAddr = addr + ai->_fileOffset;
>>> - }
>>> + });
>>> }
>>>
>>> /// \brief Set the file offset of each Atom in the section. This
>>> routine
>>> /// gets called after the linker fixes up the section offset
>>> void assignFileOffsets(uint64_t offset) override {
>>> - for (auto &ai : _atoms) {
>>> + parallel_for_each(_atoms.begin(), _atoms.end(), [&](AtomLayout
>>> *ai) {
>>> ai->_fileOffset = offset + ai->_fileOffset;
>>> - }
>>> + });
>>> }
>>>
>>> /// \brief Find the Atom address given a name, this is needed to
>>> properly
>>>
>>>
>>> _______________________________________________
>>> llvm-commits mailing list
>>> llvm-commits at cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>>
>
>
--
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