[llvm-dev] Default compression level for -compress-debug-info=zlib?

Thu Aug 2 10:23:49 PDT 2018

Hi Rui,

What's the intended advantage for compressing the debug sections? - (i)
Improved link time through smaller IO / (ii) Improved Load / startup time
for the debugger / (iii) Smaller exe with debug info for distribution /
disk space?

For i) and ii) how much this is worth it depends on balance of storage
bandwidth to compression (i) / decompression (ii) bandwidth. For spinning
drives it *might* be a win but for SATA and especially PCIe / NVMe SSD it
could be a CPU bottleneck? Though we should also bear in mind that
compression can be pipelined with writes in i) and debug info loading could
be lazy in ii)

(e.g. for highly compressible data we've generally seen ~10MiB/s output
bandwidth on single thread i7 @3.2GHz memory to memory for zlib9 with 32KiB
window, that doesn't stack up well against modern IO)

How is the compression implemented in lld? Is it chunked and therefore
paralellizable (and able to be pipelined with IO) or more serial?

I think the intention is i) so we'd be happy to link a few of our game
titles with varying compression levels vs storage types and let you know
the results. Might be a couple of weeks...

> I wonder what is the best compression level when -O2 is passed to lld.

Just to check, if the default is changed to compress at -O2 we'll still be
able to override to disable compression with -compress-debug-section=none ?

Thanks,

Simon

On Thu, Aug 2, 2018 at 7:00 AM, via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

> More data on different compression levels will be good.  In this case
> we're compressing fairly consistent looking input data (a DWARF section) so
> I think we stand a good chance of being able to pick a very reasonable
> level.
>
> I cringe at the thought of yet another user-facing knob, though.
>
> --paulr
>
>
>
> *From:* llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] *On Behalf Of *James
> Henderson via llvm-dev
> *Sent:* Thursday, August 02, 2018 6:32 AM
> *To:* Pavel Labath
> *Cc:* LLVM Dev
> *Subject:* Re: [llvm-dev] Default compression level for
> -compress-debug-info=zlib?
>
>
>
> Also not an expert, but would it make sense for this to be configurable at
> a fine-grained level, perhaps with another option, or an extension to the
> compress-debug-sections switch interface? That way users who care about the
> finer details can configure it themselves. And we should pick sensible
> options for the default.
>
>
>
> James
>
>
>
> On 2 August 2018 at 11:08, Pavel Labath via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> I don't claim to be an expert, but I did some zlib compression
> benchmarks in the past. IIRC, my conclusion from that was that the
> "DEFAULT" zlib level (6) is indeed a very good default for a lot of
> cases -- it does not generate much larger outputs, while being
> significantly faster than the max level. This all depends on the data
> set and what you intend to do with the resulting data, of course, but
> I guess my point is you don't have to choose only between 1 and 9. I
> think it would be interesting to at least get the data for the default
> level before making choice.
>
> cheers,
> pl
>
> On Thu, 2 Aug 2018 at 01:57, Rui Ueyama via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > Folks,
> >
> > I'd like to get expert's opinion on which compression level is suitable
> for lld's -compress-debug-section=zlib option, which let the linker
> compress .debug_* sections using zlib.
> >
> > Currently, lld uses compression level 9 which produces the smallest
> output in exchange for a longer link time. My question is, is this what
> people actually want? We didn't consciously choose compression level 9.
> That was just the default compression level for zlib::compress function.
> >
> > For an experiment, I created a patch to use compression level 1 instead
> of 9 and linked clang using that modified lld. By default, lld takes 1m4s
> to link clang with --compress-debug-sections=zlib. With that patch, it
> took only 31s.
> >
> > Here is a comparison of clang executable size with various
> configurations:
> >
> > no debug sections:    275 MB
> > level 9 compression:  855 MB
> > level 1 compression:  922 MB
> > no compression:      2044 MB
> >
> > Given that the best compression takes significantly longer time than the
> fastest compression, we probably should change the default to level 1. Any
> objections?
> >
> > I wonder what is the best compression level when -O2 is passed to lld.
> We could use level 9 when -O2 is passed, but is there any reason to
> compress debug sections that hard in the first place?
>
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180802/53e5f468/attachment-0001.html>