[llvm-dev] [RFC] Placing profile name data, and coverage data, outside of object files

Sean Silva via llvm-dev llvm-dev at lists.llvm.org
Sat Jul 1 12:21:16 PDT 2017

On Fri, Jun 30, 2017 at 10:39 PM, Xinliang David Li <davidxl at google.com>

> On Fri, Jun 30, 2017 at 10:25 PM, Sean Silva <chisophugis at gmail.com>
> wrote:
>> On Fri, Jun 30, 2017 at 10:04 PM, Sean Silva <chisophugis at gmail.com>
>> wrote:
>>> On Fri, Jun 30, 2017 at 5:54 PM, via llvm-dev <llvm-dev at lists.llvm.org>
>>> wrote:
>>>> Problem
>>>> -------
>>>> Instrumentation for PGO and frontend-based coverage places a large
>>>> amount of
>>>> data in object files, even though the majority of this data is not
>>>> needed at
>>>> run-time. All the data is needlessly duplicated while generating
>>>> archives, and
>>>> again while linking. PGO name data is written out into raw profiles by
>>>> instrumented programs, slowing down the training and code coverage
>>>> workflows.
>>>> Here are some numbers from a coverage + RA build of ToT clang:
>>>>   * Size of the build directory: 4.3 GB
>>>>   * Wall time needed to run "clang -help" with an SSD: 0.5 seconds
>>>>   * Size of the clang binary: 725.24 MB
>>>>   * Space wasted on duplicate name/coverage data (*.o + *.a): 923.49 MB
>>>>     - Size contributed by __llvm_covmap sections: 1.02 GB
>>>>       \_ Just within clang: 340.48 MB
>>> We live with this duplication for debug info. In some sense, if the
>>> overhead is small compared to debug info, should we even bother (i.e., we
>>> assume that users accommodate debug builds, so that is a reasonable bound
>>> on the tolerable build directory size). (I don't know the numbers; this
>>> seems pretty large so maybe it is significant compared to debug info; just
>>> saying that looking at absolute numbers is misleading here; numbers
>>> compared to debug info are a closer measure to the user's perceptions)
>>> In fact, one overall architectural observation I have is that the most
>>> complicated part of all this is simply establishing the workflow to plumb
>>> together data emitted per-TU to a tool that needs that information to do
>>> some post-processing step on the results of running the binary. That sounds
>>> a lot like the role of debug info. In fact, having a debugger open a core
>>> file is precisely equivalent to what llvm-profdata needs to do in this
>>> regard AFAICT.
>> In fact, it's so equivalent that you could in principle read the actual
>> counter values directly out of a core file. A core file could literally be
>> used as a raw profile.
>> E.g. you could in principle open the core in the debugger and then do:
>> p __profd_foo
>> p __profd_bar
>> ...
>> (and walking vprof nodes would be more complicated but doable)
>> I'm not necessarily advocating this literally be done; just showing that
>> "everything you need is there".
> A core file can be significantly larger than a raw profile data, and is
> usually truncated unless the core size limit is set. The in-process profile
> merging performance will be really bad.
Hence "in principle". Not in practice as you rightly point out.

>> Note also that the debug info approach has another nice advantage in that
>> it allows minimizing the runtime memory overhead for the program image to
>> the absolute minimum, which is important for embedded applications. Debug
>> info naturally stays out of the program image and so this problem is
>> automatically solved.
> Note that instrumented binary (not even consider coverage mapping) built
> usually does not turn on debug information. Mixing them can lead to
> significant object size increase that leads to linker failure for large
> applications.

I was imagining piggybacking on the debug info sections themselves (just
storing a blob of instrumentation data in there) as a way to naturally
transport the data. Not necessarily having any relationship between
instrumentation data and debug info. So with instrumentation enabled and
debug info disabled, we would still emit a debug section but it wouldn't
contain real debug info; it would just contain a blob of data that
instrumentation would otherwise put elsewhere.

Also, the piggyback-on-debug-info approach (or any approach trying to
externalize metadata, including Vedant's original proposal) will require
extra inputs to llvm-profdata, which may lead to workflow complexity.

We will of course want to retain the current approach where the raw
profiles are standalone as it will be preferable for initial porting
efforts and also likely the right choice in many scenarios due to
simplicity (as you point out, with IRPGO the overhead is less, so
externalizing the names isn't as much of an issue).

-- Sean Silva

> David
>> -- Sean Silva
>>> So it would be best if possible to piggyback on all the effort that has
>>> gone into plumbing that data to make debug info work. For example, I know
>>> that on Darwin there's a fair amount of system-level integration to make
>>> split dwarf "just work" while keeping debug info out of final binaries.
>>> If there is a not-too-hacky way to piggyback on debug info, that's
>>> likely to be a really slick solution. For example, debug info could in
>>> principle (if it doesn't already) contain information about the name of
>>> each counter in the counter array, so in principle it would be a complete
>>> enough description to identify each counter.
>>> I'm not very familiar with DWARF, but I'm imagining something like
>>> reserving an LLVM vendor-specific DWARF opcode/attribute/whatever and then
>>> stick a blob of data in there. Presumably we have code somewhere in LLDB
>>> that is "here's a binary, find debug info for it", and in principle we
>>> could factor out that code and lift it into an LLVM library
>>> (libFindDebugInfo) that llvm-profdata could use.
>>>>     - Size contributed by __llvm_prf_names sections: 327.46 MB
>>>>       \_ Just within clang: 106.76 MB
>>>>     => Space wasted within the clang binary: 447.24 MB
>>>> Running an instrumented clang binary triggers a 143MB raw profile write
>>>> which
>>>> is slow even with an SSD. This problem is particularly bad for
>>>> frontend-based
>>>> coverage because it generates a lot of extra name data: however, the
>>>> situation
>>>> can also be improved for PGO instrumentation.
>>>> Proposal
>>>> --------
>>>> Place PGO name data and coverage data outside of object files. This
>>>> would
>>>> eliminate data duplication in *.a/*.o files, shrink binaries, shrink raw
>>>> profiles, and speed up instrumented programs.
>>>> In more detail:
>>>> 1. The frontends get a new `-fprofile-metadata-dir=<path>` option.
>>>> This lets
>>>> users specify where llvm will store profile metadata. If the metadata
>>>> starts to
>>>> take up too much space, there's just one directory to clean.
>>>> 2. The frontends continue emitting PGO name data and coverage data in
>>>> the same
>>>> llvm::Module. So does LLVM's IR-based PGO implementation. No change
>>>> here.
>>>> 3. If the InstrProf lowering pass sees that a metadata directory is
>>>> available,
>>>> it constructs a new module, copies the name/coverage data into it,
>>>> hashes the
>>>> module, and attempts to write that module to:
>>>>   <metadata-dir>/<module-hash>.bc   (the metadata module)
>>>> If this write operation fails, it scraps the new module: it keeps all
>>>> the
>>>> metadata in the original module, and there are no changes from the
>>>> current
>>>> process. I.e with this proposal we preserve backwards compatibility.
>>> Based at my experience with Clang's implicit modules, I'm *extremely*
>>> wary of anything that might cause the compiler to emit a file that the
>>> build system cannot guess the name of. In fact, having the compiler emit a
>>> file that is not explicitly listed on the command line is basically just as
>>> bad in practice (in terms of feasibility of informing the build system
>>> about it).
>>> As a simple example, ninja simply cannot represent a dependency of this
>>> type, so if you delete a <metadata-dir>/<module-hash>.bc it won't know
>>> things need to be rebuilt (and it won't know how to clean it, etc.).
>>> So I would really strongly recommend against doing this.
>>> Again, these problems of system integration (in particular build system
>>> integration) are nasty, and if you can bypass this and piggyback on debug
>>> info then everything will "just work" because the folks that care about
>>> making sure that debugging "just works" already did the work for you.
>>> It might be more work in the short term to do the debug info approach
>>> (if it is feasible at all), but I can tell you based on the experience with
>>> implicit modules (and I'm sure you have some experience of your own) that
>>> there's just going to be a neverending tail of hitches and ways that things
>>> don't work (or work poorly) due to not having the build system / overall
>>> system integration right, so it will be worth it in the long run.
>>> -- Sean Silva
>>>> 4. Once the metadata module is written, the name/coverage data are
>>>> entirely
>>>> stripped out of the original module. They are replaced by a path to the
>>>> metadata module:
>>>>   @__llvm_profiling_metadata = "<metadata-dir>/<module-hash>.bc",
>>>>                                section "__llvm_prf_link"
>>>> This allows incremental builds to work properly, which is an important
>>>> use case
>>>> for code coverage users. When an object is rebuilt, it gets a fresh
>>>> link to a
>>>> fresh profiling metadata file. Although stale files can accumulate in
>>>> the
>>>> metadata directory, the stale files cannot ever be used.
>>>> In an IDE like Xcode, since there's just one target binary per scheme,
>>>> it's
>>>> possible to clean the metadata directory by removing the modules which
>>>> aren't
>>>> referenced by the target binary.
>>>> 5. The raw profile format is updated so that links to metadata files
>>>> are written
>>>> out in each profile. This makes it possible for all existing
>>>> llvm-profdata and
>>>> llvm-cov commands to work, seamlessly.
>>>> The indexed profile format will *not* be updated: i.e, it will contain
>>>> a full
>>>> symbol table, and no links. This simplifies the coverage mapping
>>>> reader, because
>>>> a full symbol table is guaranteed to exist before any function records
>>>> are
>>>> parsed. It also reduces the amount of coding, and makes it easier to
>>>> preserve
>>>> backwards compatibility :).
>>>> 6. The raw profile reader will learn how to read links, open up the
>>>> metadata
>>>> modules it finds links to, and collect name data from those modules.
>>>> 7. The coverage reader will learn how to read the __llvm_prf_link
>>>> section, open
>>>> up metadata modules, and lazily read coverage mapping data.
>>>> Alternate Solutions
>>>> -------------------
>>>> 1. Instead of copying name data into an external metadata module, just
>>>> copy the
>>>> coverage mapping data.
>>>> I've actually prototyped this. This might be a good way to split up
>>>> patches,
>>>> although I don't see why we wouldn't want to tackle the name data
>>>> problem
>>>> eventually.
>>>> 2. Instead of emitting links to external metadata modules, modify
>>>> llvm-cov and
>>>> llvm-profdata so that they require a path to the metadata directory.
>>>> The issue with this is that it's way too easy to read stale metadata.
>>>> It's also
>>>> less user-friendly, which hurts adoption.
>>>> 3. Use something other than llvm bitcode for the metadata module format.
>>>> Since we're mostly writing large binary blobs (compressed name data or
>>>> pre-encoded source range mapping info), using bitcode shouldn't be too
>>>> slow, and
>>>> we're not likely to get better compression with a different format.
>>>> Bitcode is also convenient, and is nice for backwards compatibility.
>>>> ------------------------------------------------------------
>>>> --------------------
>>>> If you've made it this far, thanks for taking a look! I'd appreciate any
>>>> feedback.
>>>> vedant
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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