[llvm-dev] RFC: Hotness thresholds in profile header

[Easwaran Raman via llvm-dev]

Hi,

The problem we're trying to address:

PGO transforms that are based on block counts need some way to answer
isHotBlock() query. A simple comparison of the form block_count > N, for
some constant N is not useful since even for the same program different
profile inputs can cause different answers for isHotBlock() on the same
block. This can be addressed by comparing against a reference value that
depends on the program and its input. For instance, the indexed profile
header contains MaxFunctionCount and isHotBlock() could check if the block
count exceeds a certain fraction of MaxFunctionCount.


The drawback of this approach is that it is difficult to come up with a
single fraction that works both for programs with a few hot-spots and
programs with a long tail of block execution counts. Consider the following
two profiles both with a MaxFunctionCount of 1000. In the first case, there
are many blocks with count 100 and those (along with the function with
count 1000) account for 99% of execution time. In the second case, there
are many blocks with count 1000 that together account for 99% of execution
time, but there are also a non-trivial number of blocks with count 100. In
the first case, we would like the hotness threshold to be 100 (or 10% of
MaxFunctionCount), but in the second case, we want it to be 1000 (100% of
MaxFunctionCount).


Our proposal:

We want to define hot blocks as those that account for a large
percentage(P) of execution time of the program. We then make a simplistic
assumption that all blocks take the same amount of time to execute. With
this assumption, the execution time of the program can be approximated by
the sum of execution counts of the blocks in the program. We can then
compute a maximum threshold T(P) such that execution counts of blocks whose
count is >= T(P) account for P percentage of total block count.

To illustrate, let {B1...B5} be the set of blocks whose counts are {1, 0,
2, 2, 5}. For this example:

T(50) = 5 since {B5} accounts for 50% of total block count.
T(90) = 2 since {B3, B4, B5} has a cumulative count of 9 which is 90% of
total block count.
T(95) = 1 since all blocks with non-zero block counts are needed to account
for >=95% of total block count.

We propose to record a vector of triples {P, T(P), B(P)}, where B(P) is the
number of blocks with count >= T(P), for a set of pre-defined number of
values of P into the header of the indexed profile. A possible set of
values of P are 90, 95, 99, 99.9, 99.99, but this should be configurable
through profdata tool.  The B(P) above could be useful to optimization
passes in making size/performance tradeoffs. For example, if S is the
average block size and S*B(95) fits well within cache, but S*B(99) well
exceeds the cache size, an optimization like loop unroller could choose to
use T(95) as the threshold to avoid heavy cache misses.

The {P, T(P), B(P)} triples have to be attached to the module. A common API
would be added to retrieve the set of available Ps and {T(P), B(P)} pair
for a given P for all flavors (front-end, IR level and sample) of profiles.

Thoughts?


Thanks,

Easwaran (with inputs from Teresa Johnson and David Li)
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