[LLVMdev] multithreaded performance disaster with -fprofile-instr-generate (contention on profile counters)

[Justin Bogner]

Chandler Carruth <chandlerc at google.com> writes:
> Having thought a bit about the best strategy to solve this, I think we should
> use a tradeoff of memory to reduce contention. I don't really like any of the
> other options as much, if we can get that one to work. Here is my specific
> suggestion:
>
> On Thu, Apr 17, 2014 at 5:21 AM, Kostya Serebryany <kcc at google.com> wrote:
>
>     - per-cpu counters (not portable, requires very modern kernel with lots of
>     patches)
>     - sharded counters: each counter represented as N counters sitting in
>     different cache lines. Every thread accesses the counter with index TID%N.
>     Solves the problem partially, better with larger values of N, but then
>     again it costs RAM.
>
> I think we should combine these somewhat.
>
> At an abstract level, I think we should design the profiling to support up to
> N shards of counters.
>
> I think we should have a dynamic number of shards if possible. The goal here
> is that if you never need multiple shards (single threaded) you pay
> essentially zero cost. I would have a global number of shards that changes
> rarely, and re-compute it on entry to each function with something along the
> lines of:
>
> if (thread-ID != main's thread-ID && shard_count == 1) {
>   shard_count = std::min(MAX, std::max(NUMBER_OF_THREADS, NUMBER_OF_CORES));
>   // if shard_count changed with this, we can also call a library routine here
> that does the work of allocating the actual extra shards.
> }

Is it possible to hook on something more clever than function entry?
Choosing to do this based on thread creation or something like that
could make this extra check very cheap.

> MAX is a fixed cap so even on systems with 100s of cores we don't do something
> silly. NUBER_OF_THREADS, if supported on the OS, can limit the shards when we
> only have a small number of threads in the program. NUMBER_OF_CORES, if
> supported on the OS, can limit the shards. If we don't have the number of
> threads, we just use the number of cores. If we don't have the number of
> cores, we can just guess 8 (or something).

I like the general idea of this approach. The obvious TLS approach
worried me in that it explodes memory usage by much more than you'll
generally need.

> Then, we can gracefully fall back on the following strategies to pick an index
> into the shards:
>
> - Per-core non-atomic counter updates (if we support them) using restartable
> sequences
> - Use a core-ID as the index into the shards to statistically minimize the
> contention, and do the increments atomically so it remains correct even if the
> core-ID load happens before a core migration and the counter increment occurs
> afterward
> - Use (thread-ID % number of cores) if we don't have support for getting a
> core-ID from the target OS. This will still have a reasonable distribution I
> suspect, even if not perfect.
>
> Finally, I wouldn't merge on shutdown if possible. I would just write larger
> raw profile data for multithreaded runs, and let the profdata tool merge them.

Agreed. Doing the merging offline is a clear win. The space overhead is
short lived enough that it shouldn't be a big issue.

> If this is still too much memory, then I would suggest doing the above, but
> doing it independently for each function so that only those functions actually
> called via multithreaded code end up sharding their counters.

I'd worry that doing this per-function might be too fine of granularity,
but there are certainly use cases where large chunks of the code are
only exercised by one (of many) threads, so something along these lines
could be worthwhile.

> I think this would be reasonably straightforward to implement, not
> significantly grow the cost of single-threaded instrumentation, and largely
> mitigate the contention on the counters. It can benefit from advanced hooks
> into the OS when those are available, but seems pretty easy to implement on
> roughly any OS with reasonable tradeoffs. The only really hard requirement is
> the ability to get a thread-id, but I think that is fairly reasonable (C++
> even makes this essentially mandatory).
>
> Thoughts?