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

Thu Aug 2 13:17:30 PDT 2018

On Thu, Aug 2, 2018 at 10:24 AM Simon Whittaker via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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?
>

I think (1) is definitely the case, and that's also true for a distributed
build system with which a lot of object files are copied between machines.

I doubt (2) is true. Does compressing debug sections improve debugger load
time? Of course, as you mentioned, it depends on the ratio of CPU speed and
IO speed, but since linked debug info isn't as large as the total of input
files, I think it is at least less important than (1).

As to (3), in most cases, I believe it is rare to distribute executables
with debug info widely. Only developers need debug info.

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
> ?
>

My suggestion was to use compression level 9 when both -O2 and
-compress-debug-section=zlib are specified.

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?
>>
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