Re: fio 3.2

[Jens Axboe <axboe@kernel.dk>]

On 12/03/2017 02:35 AM, Sitsofe Wheeler wrote:
> On 1 December 2017 at 07:15, Robert Elliott (Persistent Memory)
> <elliott@hpe.com> wrote:
>> While discussing NUMA, I'll mention something else I saw in Windows
>> while fixing the thread affinities there.
>>
>> At startup, fio spawns threads on all CPUs to measure the clocks
>> (fio_monotonic_clocktest).  If you've constrained the CPU affinity
>> outside fio, some of those will fail.  In Windows, something like
>> START /AFFINITY 0x55555555 fio ...
>> can cause half of the clock threads to fail.
> 
> This is very weird and doesn't make any sense (but I believe you): if
> you have multiple threads crammed on to the same CPUs the TSC no
> longer looks like it monotonically increases? Surely it should be MORE
> likely to increase because a thread is likely to be on the same CPU as
> another and can't actually be running at the same time as the other?

The threads fail to start, it's not a TSC failure. I'm guessing it's
because fio gets limited to a subset of the available CPUs, and that
causes fio to fail doing the clock check when fio_setaffinity() in
clock_thread_fn() fails.

-- 
Jens Axboe