[clangd-dev] Investigating performance tracking infrastructure

Alex L via clangd-dev clangd-dev at lists.llvm.org
Tue Aug 21 13:48:22 PDT 2018

On Tue, 21 Aug 2018 at 04:36, Kirill Bobyrev <kbobyrev.lists at gmail.com>

> Hi Alex,
> I agree with the multiple modes strategy, it would be great to enable
> tracking both sema + index and index performance without any additional
> cost, if the framework would be generic enough that would be even better!
> The other responses are inline.
> On Thu, Aug 16, 2018 at 2:04 AM Alex L <arphaman at gmail.com> wrote:
>> Thanks for your responses!
>> I realize now that I should have been more specific when it comes to
>> completion latency. We're currently interested in sema completion latency,
>> but the infrastructure that I would like to set up will support latency
>> with the completion results obtained from the index as well.
>> Essentially, for a completion test-case we would like to have the option
>> to run it in two / three modes:
>> - just sema completion
>> - index completion or sema + index completion
>> Note that we don't have to test a completion test-case in all modes, so
>> we could just have a sema based completion test.
>> This way we'll be able to identify the regressions in a particular
>> component (sema vs index) in a better way. Do you think this idea works for
>> you?
>> More responses inline:
>> On Tue, 14 Aug 2018 at 00:31, Eric Liu <ioeric at google.com> wrote:
>>> On Tue, Aug 14, 2018, 08:40 Kirill Bobyrev <kbobyrev.lists at gmail.com>
>>> wrote:
>>>> Hi Alex,
>>>> Such test-suite might be very useful and it'd be great to have it. As
>>>> Eric mentioned, I am working on pulling benchmark library into LLVM.
>>>> Although I fell behind over the past week due to the complications with
>>>> libc++ (you can follow the thread here:
>>>> http://lists.llvm.org/pipermail/llvm-dev/2018-August/125176.html).
>> Thanks! Do you a general idea of how you would like to use the
>> benchmarking library?
> I've looked into benchmark usage in libc++ and test-suite, but  they
> weren't very helpful because they seem to be very specific there. I've
> started looking into pulling the library into LLVM (
> https://reviews.llvm.org/D50894) but I have few concerns there.
>> I'm mainly interested in a more complete test that we could run using
>> some sort of harness and whose results can be fed into LNT.
> Can you please elaborate on what you mean by feeding results into LNT? Are
> you thinking about controlling the latency and failing the "benchmark
> tests" as soon as the latency is beyond some limit or are you interested in
> building LNT targets which you can run along unittests?

By feeding results I mean basically uploading and storing them in the LNT
database. We won't really need to run the test-suite using LNT, as the
performance harness will take care of it.

When it comes to the CI, we won't try to fail the tests because of the
latency while running them. We will instead run all of tests and will
upload the performance results to the store. Then we will run a follow-up
CI job that will compare the gathered results and will check for big
regressions against a baseline.

We should also have a way to run the tests locally that would check for
regressions right after the tests are done so it would be possible to do
local pre-commit testing.

>>>> Eric, Ilya and I have been discussing a possible "cheap" solution - a
>>>> tool which user can feed a compilation database and which could process
>>>> some queries (maybe in YAML format, too). This would allow a realistic
>>>> benchmark (since you could simply feed LLVM codebase or anything else with
>>>> the size you're aiming for) and be relatively easy to implement. The
>>>> downside of such approach would be that it would require some setup effort.
>>>> As an alternative, it might be worth feeding YAML symbol index instead of
>>>> the compilation commands, because currently the global symbol builder is
>>>> not very efficient. I am looking into that issue, too; we have few ideas
>>>> what the performance bottlenecks in global-symbol-builder can be and how to
>>>> fix them, hopefully I will make the tool way faster soon.
>>> Note that sema latency is something we also need to take into
>>> consideration, as it's always part of code completion flow, with or without
>>> index.
>>>> In the long term, however, I think the LLVM Community is also
>>>> interested in benchmarking other tools which exist under the LLVM umbrella,
>>>> so I think that opting in for the Benchmark approach would be more
>>>> beneficial. Having an infrastructure based on LNT that we could run either
>>>> on some buildbots or locally would be even better. The downside is that it
>>>> might turn out to be really hard to maintain a realistic test-suite, e.g.
>>>> storing YAML dump of the static index somewhere would be hard because we
>>>> wouldn't want 300+ Mb files in the tree but hosting it somewhere else and
>>>> downloading would also potentially introduce additional complexity. On the
>>>> other hand, generating a realistic index programmatically might also be
>>>> hard.
>> I don't have a strong opinion for how the index should be stored.
>> However, I think it's helpful to breakdown this problem into different
>> categories, and look at three kinds of indexing data sets:
>> - index data set that's derived from a part of the LLVM umbrella
>> (llvm/clang/test-suite/whatever).
>>   => One possible solution: this index can be rebuilt on every run.
> Yes, but that unfortunately takes too long at the moment. I started
> looking into that and fixed a YAML serialization performance problem (
> https://reviews.llvm.org/D50839), but there are few other bottlenecks
> left.
>> - index data set that's derived form a project outside of the LLVM
>> umbrella.
>>   => One possible solution: This index can be stored as an archive of
>> YAML files in one of the LLVM repos.
> That's one of doing it, right, but I'm not sure any LLVM repo would like
> to store a 300 Mb YAML file and update it over time. However, I don't know
> if there are already any cases like this and whether it might be
> acceptable.
>> - auto generated index data?
>  While this might be the most appealing option, the generation of a
> realistic index might turn out to be hard. However, I think we should have
> couple of artificial indices in the benchmarks, it might be beneficial.
>> It would probably be valuable to have different kinds of index data sets.
> Agreed, that would also mean more coverage.
> Relatively "cheap" solutions which I'm thinking about are:
> * Recording user session and mirroring the input file to Clangd to measure
> the performance. That would eliminate the complexity of creating a
> realistic benchmark without investing too much effort into the benchmark
> itself. However, it might turn out to be hard to track the performance
> contributions of individual components. Also, I'm not sure if it's generic
> enough.

Recording the user session can certainly be very appealing, but I'm not
sure how well it will translate into a realistic benchmark. I suppose it
depends on the particular performance issue. Nevertheless, It would be
really good to have this capability to help us investigate performance

> * Creating a tool which would accept YAML symbol dump, build an index and
> get a set of requests (e.g. from another file) to measure total completion
> latency. That solves most of my problems, but is tied to the index testing
> usecase which is not enough for comprehensive performance tracking.
> I unfortunately didn't get any good idea of how to build comprehensive
> performance tracking pipeline yet, e.g. how to continuously get index for
> (e.g.) LLVM, adjust buildbots, measure performance, ensure that it's
> realistic, etc.

>>>> Having said that, convenient infrastructure for benchmarking which
>>>> would align with the LNT and wouldn't require additional effort from the
>>>> users would be amazing and we are certainly interested in collaboration.
>>>> What models of the benchmarks have you considered and what do you think
>>>> about the options described above?
>> For the sema based completion latency tracking I would like to start off
>> with two simple things to get some basic infrastructure working:
>> - C++ test-case: measuring sema code-completion latency (with preamble)
>> in a file from a fixed revision of Clang.
>> - ObjC test-case: similar to above, some ObjC code with a portion.
>> One issue is that it the system headers that will be used are not static,
>> which leads to issues like the baseline might be out of date when the SDK
>> on the GreenDragon bots is updated.
>> Ideally I would use some harness based on compile commands. Each test
>> file would have a compilation command entry in the database.
>> I was also thinking that the test command could be fed into Clangd using
>> LSP itself. Similarly to how code-completion is requested in Clangd's
>> regression test, we could write a test that would send in the LSP commands
>> into Clangd. Or maybe the test harness could generate them from some sort
>> of test description (e.g. test completion at these locations at that file).
>> The latency could be measured by scanning the output of the run of Clangd
>> The test harness would then capture the result and upload it to LNT. A
>> subsequent bot would check for big regressions (e.g. +10%) against the
>> baseline (or previous result).
> Sounds good to me!

I'm hoping to put up a patch for a prototype implementation sometime this


> Kind regards,
> Kirill Bobyrev
>> Cheers,
>> Alex
>>>> Kind regards,
>>>> Kirill Bobyrev
>>>> On Tue, Aug 14, 2018 at 7:35 AM Eric Liu via clangd-dev <
>>>> clangd-dev at lists.llvm.org> wrote:
>>>>> Hi Alex,
>>>>> Kirill is working on pulling google benchmark library into llvm and
>>>>> adding benchmarks to clangd. We are also mostly interested in code
>>>>> completion latency and index performance at this point. We don't have a
>>>>> very clear idea on how to create realistic benchmarks yet e.g. what code to
>>>>> use, what static index corpus to use. I wonder if you have ideas here.
>>>>> Another option that might be worth considering is adding a tool that
>>>>> runs clangd code completion on some existing files in the llvm/clang
>>>>> codebase. It can potentially measure both code completion quality and
>>>>> latency.
>>>>> -Eric
>>>>> On Tue, Aug 14, 2018, 00:53 Alex L via clangd-dev <
>>>>> clangd-dev at lists.llvm.org> wrote:
>>>>>> Hi,
>>>>>> I'm currently investigating and putting together a plan for
>>>>>> open-source and internal performance tracking infrastructure for Clangd.
>>>>>> Initially we're interested in one particular metric:
>>>>>> - Code-completion latency
>>>>>> I would like to put together infrastructure that's based on LNT and
>>>>>> that would identify performance regressions that arise as new commits come
>>>>>> in. From the performance issues I've observed in our libclang stack the
>>>>>> existing test-suite that exist in LLVM does not really reproduce the
>>>>>> performance issues that we see in practice well enough. In my opinion we
>>>>>> should create some sort of editor performance test-suite that would be
>>>>>> unrelated to the test-suite that's used for compile time and performance
>>>>>> tracking. WDYT?
>>>>>> I'm wondering if there are any other folks looking at this at the
>>>>>> moment as well. If yes, I would like to figure out a way to collaborate on
>>>>>> a solution that would satisfy all of our requirements. Please let me know
>>>>>> if you have ideas in terms of how we should be running the tests /  what
>>>>>> the test-suite should be, or what you needs are.
>>>>>> Thanks,
>>>>>> Alex
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