[LLVMdev] RFC: Binary format for instrumentation based profiling data

[Chandler Carruth]

On Fri, Mar 28, 2014 at 1:33 PM, Justin Bogner <mail at justinbogner.com>wrote:

> Chandler Carruth <chandlerc at google.com> writes:
> >     Format 2
> >     --------
> >
> >     This format should be efficient to read and preferably reasonably
> >     compact. We'll convert from format 1 to format 2 using llvm-profdata,
> >     and clang will use format 2 for PGO.
> >
> >     Since the only particularly important operation in this use case is
> fast
> >     lookup, I propose using the on disk hash table that's currently used
> in
> >     clang for AST serialization/PTH/etc with a small amount of metadata
> in a
> >     header.
> >
> >     The hash table implementation currently lives in include/clang/Basic
> and
> >     consists of a single header. Moving it to llvm and updating the
> clients
> >     in clang should be easy. I'll send a brief RFC separately to see if
> >     anyone's opposed to moving it.
> >
> > I can mention this and we can discuss this on the other thread if you
> would
> > rather, but I'm not a huge fan of this code. My vague memory was that
> this was
> > a quick hack by Doug that he never really expected to live long-term.
>
> It may not be the prettiest piece of code, but given that it's used in
> several places in clang and hasn't needed any significant changes since
> 2010, I'd say it's fairly solid. It also has the very obvious advantage
> of already existing, which makes it a pretty good candidate for a
> version 1 format, IMHO.
>

So, I've gone and read all of it to try and get a good handle on the
current state rather than dredging up memories from so many years ago. =] I
can speak more confidently now.

The OnDiskHashTable stuff seems perfectly fine for emitting just that - an
on-disk-hash-table. However, it was never designed to support long-lived
file formats in that form. The use in serialized ASTs is specifically not
supporting a long-term file format. The biggest issue there is endianness,
and I see you've already very nicely added good support for that. The only
remaining concern I might have are the 32-bit offset limitations. While
>2gb of counter data may seem unlikely, I don't think it is inconceivable,
and even >4gb of counter data might happen. Using 64-bit offsets seems an
easy fix though. Essentially, I think this part of the code could quickly
and easily be made viable for this purpose, although it would require a bit
more cleanup and documenting the intended stability.

Anyways, the part I was truly concerned about is actually nicely factored
out -- the hashing bit. The AST's hashing is *completely* unsuitable for a
long-term file format, but my assumption is that you'd just use the
existing stable PGO hashing stuff for this table? If so, it should work
fine. If you want to hash other things (function names?), I would just urge
using something like their MD5 or some other fixed, and uncontroversial
algorithm so we don't end up wondering how a bug snuck in there N years
later. So this seems workable too.

Essentially, to answer a later question:

If you're opposed to moving the existing OnDiskHashTable into Support,
> perhaps because you don't think it should proliferate to other uses,
> the obvious alternative is to include a private copy of a stripped down
> version of it for the profile reader and writer to use themselves. I'm
> not sure if this is worth the copy pasted code, but it is an
> option. What do you think?


I think with the cleanups you've started plus a bit more documentation,
this could be a fine candidate for a generic on-disk (or, raw memory
buffer) hash table writer and reader.

OK, on to the general questions rather than ones concerning specific code...


> > I have a general preference for from-disk lookups to use tries (for
> strings,
> > prefix tries) or other fast, sorted lookup structures. They have the nice
> > property of being inherently stable and unambiguous, and not baking any
> > hashing algorithm into it.
>
> I would like to experiment with a few trie-based approaches for this as
> we try to optimize the PGO process further (both for space and for
> lookup time). Even so, it's not a sure thing that this will work better,
>

So the first question is whether it is really worth looking into other
solutions. I have a suspicion that there are better formats for this
because of one key idea: while the important operation is lookup, I don't
think it is truly *random* lookup. In fact, I suspect it will be extremely
structured lookup, with a few hot clusters of data due to similar function
"names" (where the names of things like file-local static functions get
file name prefixes and such to disambiguate them). So I think that there is
a real locality win possible in this space. Maybe not all the time, and
most of the time it may be below the noise floor, but I think it will still
be there.


> and I don't think it's worth delaying getting something that people can
> use out the door.
>

If the file format is wide open to change over the coming months, then I'm
totally down with this. Makes perfect sense. However, I get the feeling
that it isn't wide open to change, and we're going to quickly end up locked
into a particular format here to support the backwards compatibility
concerns. Personally, I'm happy to change the format *very* fluidly, at
least until there is an LLVM release, and potentially even after that, but
it would be good to hear from others that want to consume this data.
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