[lld] r232460 - [ELF] Use parallel_for_each for writing.
Rui Ueyama
ruiu at google.com
Tue Mar 17 15:10:54 PDT 2015
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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