[lld] r232460 - [ELF] Use parallel_for_each for writing.
Rafael Espíndola
rafael.espindola at gmail.com
Wed Mar 18 08:55:01 PDT 2015
Are you on Linux? What I normally do for benchmarking is
* Put all the files on tmpfs
* Disable address space randomization:
echo 0 > /proc/sys/kernel/randomize_va_space
* Disable cpu frequency scaling
for i in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor; do
echo performance > $i; done
* Use perf to run it multiple times and schedtool to run it at very
high priority:
sudo schedtool -F -p 99 -a 0x4 -e perf stat -r 20
On 17 March 2015 at 18:27, Rui Ueyama <ruiu at google.com> wrote:
> Why don't you just run it many more times?
>
> On Tue, Mar 17, 2015 at 3:20 PM, Shankar Easwaran <shankare at codeaurora.org>
> wrote:
>>
>> 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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>>>>
>>>>
>>>>
>>>>
>>>>
>>>> --
>>>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
>>>> hosted by the Linux Foundation
>>>>
>>>>
>>
>>
>> --
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted
>> by the Linux Foundation
>>
>
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