llvm dev - Aug 2018 - Default compression level for -compress-debug-info=zlib?

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>

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?
>>
>> > _______________________________________________
>> > 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
>>
>>
> _______________________________________________
> 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/aac38662/attachment.html>

> As to (3), in most cases, I believe it is rare to distribute executables
with debug info widely
>  I think it is at least less important than (1).
Agreed.
> 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.
> My suggestion was to use compression level 9 when both -O2 and
-compress-debug-section=zlib are specified.

Ok great, I'm less concerned if it still requires an explicit
-compress-debug-section=zlib
even with -O2 (I thought you were proposing to add to O2)

Still for informational / advisory purposes it would be good for us to
produce link time vs compression level vs total exe size, ideally with a
couple of different storage types (at least PCIe SSD vs spinning) and CPUs.

Thanks,

Simon

On Thu, Aug 2, 2018 at 1:17 PM, Rui Ueyama <ruiu at google.com> wrote:
> 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?
>>>
>>> > _______________________________________________
>>> > 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
>>>
>>>
>> _______________________________________________
>> 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/20180802/139721c0/attachment.html>