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

[Bob Wilson]

On Apr 16, 2014, at 12:21 AM, Chandler Carruth <chandlerc at google.com> wrote:

> 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.

It wouldn’t surprise me at all if we eventually need larger than 32-bit offsets, but I don’t think that’s a priority right now. We have almost no knowledge of the typical size of these PGO profiles, and no one is even using this new feature yet. More importantly, it would be good to share the basic OnDiskHash table implementation for clang’s serialized ASTs and modules and I’m concerned that it might be disruptive to change that right now. We’re almost certainly going to be changing the file format over time and we can coordinate a change to 64-bit offsets later if that turns out to be important.

> 
> 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.

The PGO hashing is completely unrelated. The hash here is just of the function name (or more specifically, the name that we used in the PGO data, e.g., possibly mangled). We’re currently using Bernstein’s hash function, which is so simple that I’m not worried about bugs, but it isn’t a great hash function. MD5 would make sense to me.

> 
> 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.

We need to settle of a file format ASAP for our internal work, but from the perspective of the LLVM community, it makes sense to me that this should remain wide open to change, at least until it goes out in an open-source LLVM release.

I’d like to proceed in the usual incremental fashion. This change is essentially just moving the OnDiskHashTable code from clang to llvm, and given that we seem to have agreement on the general direction, I see no reason why that can’t be committed now, along with the llvm-profdata changes to use it. It should be followed up with a change to address Chandler’s concern about the hash function.

Chandler, do you think we need to investigate the 64-bit offset issue now or can that wait until we have a better idea whether it matters?
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