[lld] r232460 - [ELF] Use parallel_for_each for writing.
Shankar Easwaran
shankare at codeaurora.org
Tue Mar 17 15:20:23 PDT 2015
Not sure if doing this same experiment on different unixes may give some
information (or) linking the same object files on windows will give more
information ?
How may data points do you usually collect ?
Shankar Easwaran
On 3/17/2015 5:10 PM, Rui Ueyama wrote:
> I reformat your results here. As you can see S/N is too low. Maybe we
> cannot say anything only from four data points.
>
> LLD with patch
> 4.16user 0.80system 0:03.06elapsed 162%CPU (0avgtext+0avgdata
> 7174160maxresident)k
> 3.94user 0.86system 0:02.93elapsed 163%CPU (0avgtext+0avgdata
> 7175808maxresident)k
> 4.36user 1.05system 0:03.08elapsed 175%CPU (0avgtext+0avgdata
> 7176320maxresident)k
> 4.17user 0.72system 0:02.93elapsed 166%CPU (0avgtext+0avgdata
> 7175120maxresident)k
>
> LLD without patch
> 4.49user 0.92system 0:03.32elapsed 162%CPU (0avgtext+0avgdata
> 7179984maxresident)k
> 4.12user 0.83system 0:03.22elapsed 154%CPU (0avgtext+0avgdata
> 7172704maxresident)k
> 4.38user 0.90system 0:03.14elapsed 168%CPU (0avgtext+0avgdata
> 7175600maxresident)k
> 4.20user 0.79system 0:03.08elapsed 161%CPU (0avgtext+0avgdata
> 7174864maxresident)k
>
>
> On Tue, Mar 17, 2015 at 2:57 PM, Shankar Easwaran <shankare at codeaurora.org>
> wrote:
>
>> 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> <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
>>
>>
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>>
>>
>>
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