[llvm-dev] Fragmented DWARF
James Henderson via llvm-dev
llvm-dev at lists.llvm.org
Thu Oct 29 03:52:39 PDT 2020
Hi Alexey,
I've just started looking at running your patch on the clang and game
packages I used for the Fragmented DWARF experiment, and on both occasions,
I got "warning: Generated debug info is broken" near the end of the link.
Digging further, the actual error this represented (for the clang case) was
"invalid e_shentsize in ELF header: 16912" (aside: there are several
Expected instances around where the former warning was reported which are
being thrown away and will cause assertions under the right configuration).
I don't really follow the code enough to understand whether this is a bug
in the code or possibly some weird interaction with our downstream patches
(I don't expect the latter, for the clang build, as our patches are
supposed to be a no-op when not using our target). I'll check what happens
with the clang package if I try using a completely vanilla LLVM with your
patch applied.
I also got a large number of "no mapping for range" warnings when linking
the game package. I tried debugging the code in the area, but the data
types are all difficult to debug, and I don't really understand the
relevant area of code enough to be able to theorise what actually is
causing this. llvm-dwarfdump --verify doesn't flag up any issues, and
there's nothing obviously broken looking at the dump of the debug data
either. Any pointers as to what might be going wrong would be appreciated.
I assume with your experiments that you build with
-ffunction-sections/-fdata-sections for maximum GC opportunities?
Thanks,
James
On Mon, 19 Oct 2020 at 09:50, James Henderson <jh7370.2008 at my.bristol.ac.uk>
wrote:
> Great, thanks Alexey! I'll try to take a look at this in the near future,
> and will report my results back here. I imagine our clang results will
> differ, purely because we probably used different toolchains to build the
> input in the first place.
>
> On Thu, 15 Oct 2020 at 10:08, Alexey Lapshin <avl.lapshin at gmail.com>
> wrote:
>
>>
>> On 13.10.2020 10:20, James Henderson wrote:
>>
>> The script included in the patch can be used to convert an object
>> containing normal DWARF into an object using fragmented DWARF. It does this
>> by using llvm-dwarfdump to dump the various sections, parses the output to
>> identify where it should split (using the offsets of the various entries),
>> and then writes new section headers accordingly - you can see roughly what
>> it's doing if you get a chance to watch the talk recording. The additional
>> section headers are appended to the end of the ELF section header table,
>> whilst the original DWARF is left in the same place it was before (making
>> use of the fact that section headers don't have to appear in offset order).
>> The script also parses and fragments the relocation sections targeting the
>> DWARF sections so that they match up with the fragmented DWARF sections.
>> This is clearly all suboptimal - in practice the compiler should be
>> modified to do the fragmenting upfront, to save having to parse a tool's
>> stdout, but that was just the simplest thing I could come up with to
>> quickly write the script. Full details of the script usage are included in
>> the patch description, if you want to play around with it.
>>
>> If Alexey could point me at the latest version of his patch, I'd be happy
>> to run that through either or both of the packages I used to see what
>> happens. Equally, I'd be happy if Alexey is able to run my script to
>> fragment and measure the performance of a couple of projects he's been
>> working with. Based purely on the two packages I've tried this with, I can
>> tell already that the results can vary wildly. My expectation is that
>> Alexey's approach will be slower (at least in its current form, but
>> probably more generally), but produce smaller output, but to what scale I
>> have no idea.
>>
>> James, I updated the patch - https://reviews.llvm.org/D74169.
>>
>> To make it working it is necessary to build example with
>> -ffunction-sections and specify following options to the linker :
>>
>> --gc-sections --gc-debuginfo --gc-debuginfo-no-odr
>>
>> For clang binary I got following results:
>>
>> 1. --gc-sections = binary size 1,5G, Debug Info size(*)1.2G
>>
>> 2. --gc-sections --gc-debuginfo = binary size 840M, 8x performance
>> decrease, Debug Info size 542M
>>
>> 3. --gc-sections --gc-debuginfo --gc-debuginfo-no-odr = binary size 1,3G,
>> 16x performance decrease, Debug Info size 1G
>>
>> (*) .debug_info+.debug_str+.debug_line+.debug_ranges+.debug_loc
>>
>>
>> I added option --gc-debuginfo-no-odr, so that size reduction could be
>> compared correctly. Without that option D74169 does types deduplication and
>> then it is not correct to compare resulting size with "Fragmented DWARF"
>> solution which does not do types deduplication.
>>
>> Also, I look at your D89229 <https://reviews.llvm.org/D89229> and would
>> share results some time later.
>>
>> Thank you, Alexey.
>>
>>
>> I think linkers parse .eh_frame partly because they have no other choice.
>> That being said, I think it's format is not too complex, so similarly the
>> parser isn't too complex. You can see LLD's ELF implementation in
>> ELF/EhFrame.cpp, how it is used in ELF/InputSection.cpp (see the bits to do
>> with EhInputSection) and EhFrameSection in ELF/SyntheticSections.h (plus
>> various usages of these two throughout the LLD code). I think the key to
>> any structural changes in the DWARF format to make them more amenable to
>> link-time parsing is being able to read a minimal amount without needing to
>> parse the payload (e.g. a length field, some sort of type, and then using
>> the relocations to associate it accordingly).
>>
>> James
>>
>> On Mon, 12 Oct 2020 at 20:48, David Blaikie <dblaikie at gmail.com> wrote:
>>
>>> Awesome! Sorry I missed the lightning talk, but really interested to see
>>> this sort of thing (though it's not directly/immediately applicable to the
>>> use case I work with - Split DWARF, something similar could be used there
>>> with further work)
>>>
>>> Though it looks like the patch has mostly linker changes - where/how do
>>> you generate the fragmented DWARF to begin with? Via the Python script? Run
>>> over assembly? I'd be surprised if it was achievable that way - curious to
>>> know more.
>>>
>>> Got a rough sense/are you able to run apples-to-apples comparisons with
>>> Alexey's linker-based patches to compare linker time/memory overhead versus
>>> resulting output size gains?
>>>
>>> (& yeah, I'm a bit curious about how the linkers do eh_frame rewriting,
>>> if the format is especially amenable to a lightweight parsing/rewriting and
>>> how we could make the DWARF more amenable to that too)
>>>
>>> On Mon, Oct 12, 2020 at 6:41 AM James Henderson <
>>> jh7370.2008 at my.bristol.ac.uk> wrote:
>>>
>>>> Hi all,
>>>>
>>>> At the recent LLVM developers' meeting, I presented a lightning talk on
>>>> an approach to reduce the amount of dead debug data left in an executable
>>>> following operations such as --gc-sections and duplicate COMDAT removal. In
>>>> that presentation, I presented some figures based on linking a game that
>>>> had been built by our downstream clang port and fragmented using the
>>>> described approach. Since recording the presentation, I ran the same
>>>> experiment on a clang package (this time built with a GCC version). The
>>>> comparable figures are below:
>>>>
>>>> Link-time speed (s):
>>>>
>>>> +--------------------+-------+---------------+------+------+------+------+------+
>>>> | Package variant | No GC | GC 1 (normal) | GC 2 | GC 3 | GC 4 | GC
>>>> 5 | GC 6 |
>>>>
>>>> +--------------------+-------+---------------+------+------+------+------+------+
>>>> | Game (plain) | 4.5 | 4.9 | 4.2 | 3.6 | 3.4 |
>>>> 3.3 | 3.2 |
>>>> | Game (fragmented) | 11.1 | 11.8 | 9.7 | 8.6 | 7.9 |
>>>> 7.7 | 7.5 |
>>>> | Clang (plain) | 13.9 | 17.9 | 17.0 | 16.7 | 16.3 |
>>>> 16.2 | 16.1 |
>>>> | Clang (fragmented) | 18.6 | 22.8 | 21.6 | 21.1 | 20.8 |
>>>> 20.5 | 20.2 |
>>>>
>>>> +--------------------+-------+---------------+------+------+------+------+------+
>>>>
>>>> Output size - Game package (MB):
>>>>
>>>> +---------------------+-------+------+------+------+------+------+------+
>>>> | Category | No GC | GC 1 | GC 2 | GC 3 | GC 4 | GC 5 | GC 6
>>>> |
>>>>
>>>> +---------------------+-------+------+------+------+------+------+------+
>>>> | Plain (total) | 1149 | 1121 | 1017 | 965 | 938 | 930 | 928
>>>> |
>>>> | Plain (DWARF*) | 845 | 845 | 845 | 845 | 845 | 845 | 845
>>>> |
>>>> | Plain (other) | 304 | 276 | 172 | 120 | 93 | 85 | 82
>>>> |
>>>> | Fragmented (total) | 1044 | 940 | 556 | 373 | 287 | 263 | 255
>>>> |
>>>> | Fragmented (DWARF*) | 740 | 664 | 384 | 253 | 194 | 178 | 173
>>>> |
>>>> | Fragmented (other) | 304 | 276 | 172 | 120 | 93 | 85 | 82
>>>> |
>>>> +---------------------+-------+------+------+------+------+------+------+
>>>>
>>>>
>>>> Output size - Clang (MB):
>>>>
>>>> +---------------------+-------+------+------+------+------+------+------+
>>>> | Category | No GC | GC 1 | GC 2 | GC 3 | GC 4 | GC 5 | GC 6
>>>> |
>>>>
>>>> +---------------------+-------+------+------+------+------+------+------+
>>>> | Plain (total) | 2596 | 2546 | 2406 | 2332 | 2293 | 2273 | 2251
>>>> |
>>>> | Plain (DWARF*) | 1979 | 1979 | 1979 | 1979 | 1979 | 1979 | 1979
>>>> |
>>>> | Plain (other) | 616 | 567 | 426 | 353 | 314 | 294 | 272
>>>> |
>>>> | Fragmented (total) | 2397 | 2346 | 2164 | 2069 | 2017 | 1990 | 1963
>>>> |
>>>> | Fragmented (DWARF*) | 1780 | 1780 | 1738 | 1716 | 1703 | 1696 | 1691
>>>> |
>>>> | Fragmented (other) | 616 | 567 | 426 | 353 | 314 | 294 | 272
>>>> |
>>>>
>>>> +---------------------+-------+------+------+------+------+------+------+
>>>>
>>>> *DWARF size == total size of .debug_info + .debug_line + .debug_ranges
>>>> + .debug_aranges + .debug_loc
>>>>
>>>> Additionally, I have posted https://reviews.llvm.org/D89229 which
>>>> provides the python script and linker patches used to reproduce the above
>>>> results on my machine. The GC 1/2/3/4/5/6 correspond to the linker option
>>>> added in that patch --mark-live-pc with values 1/0.8/0.6/0.4/0.2/0
>>>> respectively.
>>>>
>>>> During the conference, the question was asked what the memory usage and
>>>> input size impact was. I've summarised these below:
>>>>
>>>> Input file size total (GB):
>>>> +--------------------+------------+
>>>> | Package variant | Total Size |
>>>> +--------------------+------------+
>>>> | Game (plain) | 2.9 |
>>>> | Game (fragmented) | 4.2 |
>>>> | Clang (plain) | 10.9 |
>>>> | Clang (fragmented) | 12.3 |
>>>> +--------------------+------------+
>>>>
>>>> Peak Working Set Memory usage (GB):
>>>> +--------------------+-------+------+
>>>> | Package variant | No GC | GC 1 |
>>>> +--------------------+-------+------+
>>>> | Game (plain) | 4.3 | 4.7 |
>>>> | Game (fragmented) | 8.9 | 8.6 |
>>>> | Clang (plain) | 15.7 | 15.6 |
>>>> | Clang (fragmented) | 19.4 | 19.2 |
>>>> +--------------------+-------+------+
>>>>
>>>> I'm keen to hear what people's feedback is, and also interested to see
>>>> what results others might see by running this experiment on other input
>>>> packages. Also, if anybody has any alternative ideas that meet the goals
>>>> listed below, I'd love to hear them!
>>>>
>>>> To reiterate some key goals of fragmented DWARF, similar to what I said
>>>> in the presentation:
>>>> 1) Devise a scheme that gives significant size savings without being
>>>> too costly. It's clear from just the two packages I've tried this on that
>>>> there is a fairly hefty link time performance cost, although the exact cost
>>>> depends on the nature of the input package. On the other hand, depending on
>>>> the nature of the input package, there can also be some big gains.
>>>> 2) Devise a scheme that doesn't require any linker knowledge of DWARF.
>>>> The current approach doesn't quite achieve this properly due to the slight
>>>> misuse of SHF_LINK_ORDER, but I expect that a pivot to using non-COMDAT
>>>> group sections should solve this problem.
>>>> 3) Provide some kind of halfway house between simply writing tombstone
>>>> values into dead DWARF and fully parsing the DWARF to reoptimise
>>>> its/discard the dead bits.
>>>>
>>>> I'm hopeful that changes could be made to the linker to improve the
>>>> link-time cost. There seems to be a significant amount of the link time
>>>> spent creating the input sections. An alternative would be to devise a
>>>> scheme that would avoid the literal splitting into section headers, in
>>>> favour of some sort of list of split-points that the linker uses to split
>>>> things up (a bit like it already does for .eh_frame or mergeable sections).
>>>>
>>>
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