Jon? Are yuo going to review this one? Thanks, Corinna
On Jan 20 00:18, Mark Geisert wrote: > Commentary wording now refers to tasks (i.e., threads) rather than > processes. This makes it somewhat easier to justify adding two kinds of > counters together. After researching what "load average" has meant over > time, we have what seems like a reasonable implementation, modulo > Windows differences to Linux. The best resource I found is: > https://www.brendangregg.com/blog/2017-08-08/linux-load-averages.html > > At end of load_init(), obtain and discard the first measure of the > counters to deal with the first call always returning error, no data. > Follow this with a specific short delay so the next measure actually has > data to report. > > At least one older version of Windows, i.e. Win10 Pro 21H1, has a different > name/location for the '% Processor Time' counter and is missing the > 'Processor Queue Length' counter entirely. Code is changed to support > both possible locations of the former and treat the missing latter as always > reporting 0.0. > > A release note is added for 3.5.6. > > Reported-by: Mark Liam Brown <brownmarkl...@gmail.com> > Addresses: https://cygwin.com/pipermail/cygwin/2024-August/256361.html > Signed-off-by: Mark Geisert <m...@maxrnd.com> > Fixes: 4dc982ddf60b (Cygwin: loadavg: improve debugging of load_init) > > --- > winsup/cygwin/loadavg.cc | 50 ++++++++++++++++++++++++++----------- > winsup/cygwin/release/3.5.6 | 3 +++ > 2 files changed, 39 insertions(+), 14 deletions(-) > > diff --git a/winsup/cygwin/loadavg.cc b/winsup/cygwin/loadavg.cc > index 127591a2e..1c5e553a9 100644 > --- a/winsup/cygwin/loadavg.cc > +++ b/winsup/cygwin/loadavg.cc > @@ -15,17 +15,23 @@ > > A global load average estimate is maintained in shared memory. Access to > that > is guarded by a mutex. This estimate is only updated at most every 5 > seconds. > + The updates are done by any/all callers of the getloadavg() syscall. > > - We attempt to count running and runnable processes, but unlike linux we > don't > - count processes in uninterruptible sleep (blocked on I/O). > + We attempt to count running and runnable tasks (i.e., threads), but unlike > + Linux we don't count tasks in uninterruptible sleep (blocked on I/O). > There > + doesn't seem to be a kernel counter for the latter on Windows. > > - The number of running processes is estimated as (NumberOfProcessors) * (% > - Processor Time). The number of runnable processes is estimated as > - ProcessorQueueLength. > + In the following text and code, "PDH" refers to Process Data Helper, a > + Windows component that arranges access to kernel counters. > + > + The number of running tasks is estimated as > + (the NumberOfProcessors counter) * (the % Processor Time counter). > + The number of runnable tasks is taken to be the ProcessorQueueLength > counter. > > Note that PDH will only return data for '% Processor Time' afer the second > call to PdhCollectQueryData(), as it's computed over an interval, so the > first > - attempt to estimate load will fail and 0.0 will be returned. > + attempt to estimate load will fail and 0.0 will be returned. (This > nuisance > + is now worked-around near the end of load_init() below.) > > We also assume that '% Processor Time' averaged over the interval since the > last time getloadavg() was called is a good approximation of the > instantaneous > @@ -68,8 +74,19 @@ static bool load_init (void) > if (status != STATUS_SUCCESS) > { > debug_printf ("PdhAddEnglishCounterW(time), status %y", status); > - return false; > + > + /* Windows 10 Pro 21H1, and maybe others, use an alternative name */ > + status = PdhAddEnglishCounterW (query, > + L"\\Processor Information(_Total)\\% Processor Time", > + 0, &counter1); > + if (status != STATUS_SUCCESS) > + { > + debug_printf ("PdhAddEnglishCounterW(alt time), status %y", > status); > + return false; > + } > } > + > + /* Windows 10 Pro 21H1, and maybe others, are missing this counter */ > status = PdhAddEnglishCounterW (query, > L"\\System\\Processor Queue Length", > 0, &counter2); > @@ -77,7 +94,7 @@ static bool load_init (void) > if (status != STATUS_SUCCESS) > { > debug_printf ("PdhAddEnglishCounterW(queue length), status %y", status); > - return false; > + ; /* Ignore missing counter, just use zero in later calculations */ > } > > mutex = CreateMutex(&sec_all_nih, FALSE, "cyg.loadavg.mutex"); > @@ -87,6 +104,12 @@ static bool load_init (void) > } > > initialized = true; > + > + /* Do the first data collection (which always fails) here, rather than in > + get_load(). We wait at least one tick afterward so the collection done > + in get_load() is guaranteed to have data to work with. */ > + (void) PdhCollectQueryData (query); /* ignore errors */ > + Sleep (15/*ms*/); /* wait for at least one kernel tick to have occurred > */ > } > > return initialized; > @@ -101,8 +124,8 @@ static bool get_load (double *load) > if (ret != ERROR_SUCCESS) > return false; > > - /* Estimate the number of running processes as (NumberOfProcessors) * (% > - Processor Time) */ > + /* Estimate number of running tasks as > + (NumberOfProcessors) * (% Processor Time) */ > PDH_FMT_COUNTERVALUE fmtvalue1; > ret = PdhGetFormattedCounterValue (counter1, PDH_FMT_DOUBLE, NULL, > &fmtvalue1); > if (ret != ERROR_SUCCESS) > @@ -110,11 +133,10 @@ static bool get_load (double *load) > > double running = fmtvalue1.doubleValue * wincap.cpu_count () / 100; > > - /* Estimate the number of runnable processes using ProcessorQueueLength */ > - PDH_FMT_COUNTERVALUE fmtvalue2; > + /* Estimate the number of runnable tasks as ProcessorQueueLength */ > + PDH_FMT_COUNTERVALUE fmtvalue2 = { 0 }; > ret = PdhGetFormattedCounterValue (counter2, PDH_FMT_LONG, NULL, > &fmtvalue2); > - if (ret != ERROR_SUCCESS) > - return false; > + /* Ignore any error accessing this counter, just treat as if zero was read > */ > > LONG rql = fmtvalue2.longValue; > > diff --git a/winsup/cygwin/release/3.5.6 b/winsup/cygwin/release/3.5.6 > index 4ccf94e38..5322be6d7 100644 > --- a/winsup/cygwin/release/3.5.6 > +++ b/winsup/cygwin/release/3.5.6 > @@ -13,3 +13,6 @@ Fixes: > > - Add fd validation where needed in mq_* functions. > Addresses: https://cygwin.com/pipermail/cygwin/2025-January/257090.html > + > +- Minor updates to load average calculations. > + Addresses: https://cygwin.com/pipermail/cygwin/2024-August/256361.html > -- > 2.45.1
