[PATCH] D22438: [llvm-profdata] Speed up merging by using a thread pool

[Vedant Kumar via llvm-commits]

vsk added a comment.

In https://reviews.llvm.org/D22438#486320, @silvas wrote:

> The way you arrived at sqrt(N) seems sketchy to me.


I didn't consider the option of splitting the input by twos. You're right that it's possible to do better than T - 1 + (N/T) serial merges.

> Assuming infinite parallelism, the first step performs N/2 merges and then you recurse on the N/2 merged profiles [...]. Which must work out to something like log(N) serial merges.


In practice (dropping the assumption of infinite parallelism), I don't think we can get to log(N) serial merges. You have to at least visit each input once (using any one of T threads), so N/T serial operations looks like the lower bound.

That said, your suggested approach seems better and I'll take a shot at it.

In https://reviews.llvm.org/D22438#486320, @davidxl wrote:

> There isn't any reduction of the number of merges compared with serial case. Say there are T threads, the total number of merges is: T * f(N/T) + T - 1, so f(N) is still N-1.


No disagreement here. We will count the same (or higher) number of merges in total when using T threads. It's strictly meant as a speed improvement.

> I suspect a much larger speed up can be obtained by unifying symbol table creation across profiles (that are generated from the same binary -- as their symtab is identical). I was planning doing that but did not find time. If you can help with that, that will be a great win.


That sounds like a great idea. Did you have in mind some sort of cache of InstrProfSymtabs which can optionally be passed to the raw profile reader? I can investigate that after this patch is done.


https://reviews.llvm.org/D22438