On Thu, 5 May 2022 at 07:56, Alexandre Oliva via Libstdc++ <libstd...@gcc.gnu.org> wrote: > > > The async call and future variable initialization may take a while to > complete on uniprocessors, especially if the async call and other > unrelated processes run before context switches back to the main > thread. > > Taking steady_begin only then sometimes causes the 11*100ms in the > slow clock, counted from before the async call, to not be enough for > the measured wait to last 1s in the steady clock. I've seen it fall > short of 1s by as little as a third of a tenth of a second in some > cases, but in one surprisingly extreme case the elapsed wait time got > only up to 216.7ms. > > Initializing both timestamps next to each other, before the async > call, appears to avoid the problem entirely. I've renamed the > variable moved out of the block so as to avoid name hiding in the > subsequent block, that has another steady_begin variable. > > The second wait fails a lot less frequently, but the 2s limit has been > exceeded, so I'm bumping up the max sleep to ~4s, and the tolerance to > 3s. > > > I wasn't sure about whether to leave the added outputs that I put in to > confirm the failure modes. Please let me know in case they're > undersirable, and I'll take them out. > > Regstrapped on x86_64-linux-gnu, ppc64le-linux-gnu, and also tested on > ppc- and ppc64-vx7r2. Ok to install?
Hi Alex, This one slipped through the cracks, sorry. Leaving the outputs seems useful in this case. For timing-sensitive tests like this it's useful to have the output for exactly how long it took when there's a FAIL in the logs. The patch is OK for trunk now (and should still apply cleanly). > > > for libstdc++-v3/ChangeLog > > * testsuite/30_threads/async/async.cc (test04): Initialize > steady_start, renamed from steady_begin, next to slow_start. > Increase tolerance for final wait. > --- > libstdc++-v3/testsuite/30_threads/async/async.cc | 17 ++++++++++++----- > 1 file changed, 12 insertions(+), 5 deletions(-) > > diff --git a/libstdc++-v3/testsuite/30_threads/async/async.cc > b/libstdc++-v3/testsuite/30_threads/async/async.cc > index 38943ff1a9a5e..a36e1aee8bdef 100644 > --- a/libstdc++-v3/testsuite/30_threads/async/async.cc > +++ b/libstdc++-v3/testsuite/30_threads/async/async.cc > @@ -20,6 +20,7 @@ > // with this library; see the file COPYING3. If not see > // <http://www.gnu.org/licenses/>. > > +#include <iostream> > > #include <future> > #include <thread> > @@ -133,6 +134,7 @@ void test04() > { > using namespace std::chrono; > > + auto const steady_start = steady_clock::now(); > auto const slow_start = slow_clock::now(); > future<void> f1 = async(launch::async, []() { > std::this_thread::sleep_for(std::chrono::seconds(2)); > @@ -140,21 +142,26 @@ void test04() > > // Wait for ~1s > { > - auto const steady_begin = steady_clock::now(); > auto const status = f1.wait_until(slow_start + milliseconds(100)); > VERIFY(status == std::future_status::timeout); > - auto const elapsed = steady_clock::now() - steady_begin; > + auto const elapsed = steady_clock::now() - steady_start; > + if (elapsed < seconds(1)) > + std::cout << elapsed.count () << "ns < 1s" << std::endl; > VERIFY(elapsed >= seconds(1)); > VERIFY(elapsed < seconds(2)); > } > > - // Wait for up to ~2s more > + // Wait for up to ~4s more, but since the async sleep completes, the > + // actual wait may be shorter than 1s. Tolerate 3s because 2s > + // hasn't been enough in some extreme cases. > { > auto const steady_begin = steady_clock::now(); > - auto const status = f1.wait_until(slow_start + milliseconds(300)); > + auto const status = f1.wait_until(slow_start + milliseconds(500)); > VERIFY(status == std::future_status::ready); > auto const elapsed = steady_clock::now() - steady_begin; > - VERIFY(elapsed < seconds(2)); > + if (elapsed >= seconds(3)) > + std::cout << elapsed.count () << "ns > 2s" << std::endl; > + VERIFY(elapsed < seconds(3)); > } > } > > > > -- > Alexandre Oliva, happy hacker https://FSFLA.org/blogs/lxo/ > Free Software Activist GNU Toolchain Engineer > Disinformation flourishes because many people care deeply about injustice > but very few check the facts. Ask me about <https://stallmansupport.org> >
