[llvm-dev] [PGO] Thoughts on adding a key-value store to profile data formats

[Xinliang David Li via llvm-dev]

Tagging profile data with such information is generally useful. My thoughts are

1) such information is probably not needed to be stored in raw format
profile data -- so no runtime changes are needed -- only llvm-profdata
and indexed format need to be enhanced to support this.
2) A more general way is just add an option:
--embed_label=<customized_label>, where the label is a string can be
key/value pairs encoded in user's favorite format. The format of the
key-value pairs are not specified and remain opaque to Instr/Sample
Profiler
3) labels from multiple source profiles will be merged when merge
command is used.

On Fri, Jan 15, 2016 at 11:06 AM, Nathan Slingerland <slingn at gmail.com> wrote:
> Hi all,
>
> I'd liked to get your thoughts on possibly adding a generic key-value store
> to the profile data formats for 'metadata'. Some potential uses cases:
>
> I. Profile Features
>
> The most basic use could be as a central repository for internal bits of
> housekeeping information about the profile data. For example, to
> differentiate between FE and IR instrumentation:
>
>   llvm.instrumentation_source: "IR"
>
> A key-value store would make it simple to add new bits of information and
> help keep everything human-readable for the text-based test formats. This
> could potentially also help with error checking at the llvm-profdata level
> if the Reader classes exposed it.
>

This is ok to have, but I don't think the reader class should rely on
meta data to make decisions (as meta data can be thrown away without
affecting correctness). Formal approach such as the one proposed (to
encode it in variant bits of the version field) should be used.


> II. Profile Context
>
> Basic (lightweight) information about the profile could be automatically
> gathered at profile time. The idea would be to automatically label profiles
> with contextual information so that the age/origin of a profile could be
> inspected using the llvm-profdata tool.
>
>   $ llvm-profdata show -metadata foo.profdata
>   llvm.profile_start_time: "2016-01-08T23:41:56.755Z"
>   llvm.profile_duration: 5.102s
>   llvm.exe_time: "2016-01-08T23:35:56.745Z"

Other examples include options and workload used in the training run.

>   Total functions: 4
>   Maximum function count: 866988873
>   Maximum internal block count: 267914296
>
> Other possibilities: executable path, command line arguments, system info
> (uname)

yes.

>
> III. Custom Content
>
> The key-value store itself could be exposed to developers via the
> llvm-profdata tool. This would allow for users to associate arbitrary custom
> data with a profile, as well as inspect it:
>
>   $ llvm-profdata merge -metadata=customkey,value1 foo.profraw -o
> foo.profdata
>   $ llvm-profdata show -metadata foo.profdata
>   customkey: "value1"
>   Total functions: 4
>   Maximum function count: 866988873
>   Maximum internal block count: 267914296
>
> Developers could add as much custom context as they find valuable:

I think all meta data should be custom defined -- the profile reader
should not need to understand them.


>
>   $ llvm-profdata merge -metadata="mysoft.version,${SOFTWARE_VERSION}
> (${BUILD_NUMBER})" -metadata="mysoft.exe_md5,`md5 -q foo.exe` foo.profraw -o
> foo.profdata
>   $ llvm-profdata show -metadata foo.profdata
>   mysoft.version: "0.1.0"
>   mysoft.exe_md5: "337b5c5bc29cbdca090a1921a58465d6"
>   Total functions: 4
>   Maximum function count: 866988873
>   Maximum internal block count: 267914296
>
> Other information that might be interesting: git/svn revision, workload
> description, system info (uname -a)
>
> This would be a way to embed almost any platform-specific or heavy-weight
> data without requiring the addition of platform-specific code in compiler-rt
> and without impacting other developers.
>

yes.

>
> When profiles are merged it might be simplest to keep all input metadata
> (machine-readable things such as feature bits might need to be handled
> differently):

Feature bits should not be part of it.

>
>   $ llvm-profdata merge -weighted-input=3,foo.profdata bar.profdata -o
> foobar.profdata
>   $ llvm-profdata show -metadata foobar.profdata
>   foo.profdata
>     llvm.profile_weight: 3
>     llvm.profile_start_time: "2016-01-08T23:41:56.755Z"
>     llvm.profile_duration: 5.102s
>     llvm.exe_time: "2016-01-08T23:35:56.745Z"
>     customkey: "value1"
>   bar.profdata
>     llvm.profile_weight: 1
>     llvm.profile_start_time: "2016-01-15T00:08:41.168Z"
>     llvm.profile_duration: "1.001s"
>     llvm.exe_time: "2016-01-15T00:08:13.000Z"
>     customkey: "value2"
>   Total functions: 4
>   Maximum function count: 866988873
>   Maximum internal block count: 267914296
>
> In terms of implementation, the metadata could live as a separate contiguous
> section in the binary profile formats. It might make sense to encode it in
> something like YAML so that it could also be directly embedded in the
> various text formats.
>

A single string after the header should do.

thanks,

David

> ----
>
> What do you think? How useful would any of the above be to you or other PGO
> users?
> Can you think of any other use cases?
>
> -Nathan