From 63078a04984b73e1fdeb4571a63605ee5c13f929 Mon Sep 17 00:00:00 2001 From: Tom Tromey Date: Tue, 13 Dec 2022 12:03:34 -0700 Subject: Avoid submitting empty tasks in parallel_for_each I found that parallel_for_each would submit empty tasks to the thread pool. For example, this can happen if the number of tasks is smaller than the number of available threads. In the DWARF reader, this resulted in the cooked index containing empty sub-indices. This patch arranges to instead shrink the result vector and process the trailing entries in the calling thread. --- gdbsupport/parallel-for.h | 30 ++++++++++++++++++++++++++++++ 1 file changed, 30 insertions(+) (limited to 'gdbsupport') diff --git a/gdbsupport/parallel-for.h b/gdbsupport/parallel-for.h index b565676a0d0..de9ebb15746 100644 --- a/gdbsupport/parallel-for.h +++ b/gdbsupport/parallel-for.h @@ -70,6 +70,12 @@ public: return result; } + /* Resize the results to N. */ + void resize (size_t n) + { + m_futures.resize (n); + } + private: /* A vector of futures coming from the tasks run in the @@ -108,6 +114,12 @@ public: } } + /* Resize the results to N. */ + void resize (size_t n) + { + m_futures.resize (n); + } + private: std::vector> m_futures; @@ -232,6 +244,24 @@ parallel_for_each (unsigned n, RandomIt first, RandomIt last, end = j; remaining_size -= chunk_size; } + + /* This case means we don't have enough elements to really + distribute them. Rather than ever submit a task that does + nothing, we short-circuit here. */ + if (first == end) + end = last; + + if (end == last) + { + /* We're about to dispatch the last batch of elements, which + we normally process in the main thread. So just truncate + the result list here. This avoids submitting empty tasks + to the thread pool. */ + count = i; + results.resize (count); + break; + } + if (parallel_for_each_debug) { debug_printf (_("Parallel for: elements on worker thread %i\t: %zu"), -- cgit v1.2.1