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io_tester: implement latency correction for jobs with RPS #2260

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io_tester: implement latency correction for jobs with RPS #2260

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Commits on May 22, 2024

  1. io_tester: implement latency correction for jobs with RPS 

    Currently, for jobs with RPS (request-per-second) rate specified,
io_tester measures only service time. It means, that if servicing
one of requests takes much longer than available delta time between
consecutive requests (issuing the next request is delayed), then
the measured latency does not depict that fact. In such case we
issue less requests than required and high latency is reported just
for the first request.

For instance, if there is a latency spike for one of requests, that
exceeds the available time of service according to RPS schedule, then
the total number of scheduled requests does not match the expected
count calculated as 'TOTAL = duration_seconds * RPS'.

Furthermore, the percentiles with latency printed at the end of the
simulation may show inaccurate data. Firstly, the count of samples
is lower than expected. Secondly, if the amount of time needed to
handle requests after the latency spike returned to the ordinary
value, then our statistics show that handling of only one request
was long, but it is not true - io_tester stopped sending requests
at given RPS and this way other requests could not be properly measured.
This indicates that io_tester suffers from coordinated omission problem.

This change implements latency correction flag. When it is enabled,
then io_tester measures the total request latency including the delay
between the expected schedule time and the actual schedule time.
Moreover, if any of requests take more time than available, then
io_tester tries to schedule 'delayed' requests as soon as possible
to return to the defined schedule.

Signed-off-by: Patryk Wrobel <[email protected]>
    @pwrobelse
    pwrobelse committed May 22, 2024
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