- clarify how submission latency is calculated for async ioengines - it is slat (not clat) that is near zero for sync ioengines - Note that submission latency + completion latency = total latency Signed-off-by: Vincent Fu <vincent.fu@xxxxxxxxxxx> --- HOWTO.rst | 29 ++++++++++++++++++----------- 1 file changed, 18 insertions(+), 11 deletions(-) diff --git a/HOWTO.rst b/HOWTO.rst index 28ac2b7c..470777e2 100644 --- a/HOWTO.rst +++ b/HOWTO.rst @@ -4165,24 +4165,31 @@ writes in the example above). In the order listed, they denote: **slat** Submission latency (**min** being the minimum, **max** being the maximum, **avg** being the average, **stdev** being the standard - deviation). This is the time it took to submit the I/O. For - sync I/O this row is not displayed as the slat is really the - completion latency (since queue/complete is one operation there). - This value can be in nanoseconds, microseconds or milliseconds --- - fio will choose the most appropriate base and print that (in the - example above nanoseconds was the best scale). Note: in :option:`--minimal` mode - latencies are always expressed in microseconds. + deviation). This is the time from when fio initialized the I/O + to submission. For synchronous ioengines this includes the time + up until just before the ioengine's queue function is called. + For asynchronous ioengines this includes the time up through the + completion of the ioengine's queue function (and commit function + if it is defined). For sync I/O this row is not displayed as the + slat is negligible. This value can be in nanoseconds, + microseconds or milliseconds --- fio will choose the most + appropriate base and print that (in the example above + nanoseconds was the best scale). Note: in :option:`--minimal` + mode latencies are always expressed in microseconds. **clat** Completion latency. Same names as slat, this denotes the time from - submission to completion of the I/O pieces. For sync I/O, clat will - usually be equal (or very close) to 0, as the time from submit to - complete is basically just CPU time (I/O has already been done, see slat - explanation). + submission to completion of the I/O pieces. For sync I/O, this + represents the time from when the I/O was submitted to the + operating system to when it was completed. For asynchronous + ioengines this is the time from when the ioengine's queue (and + commit if available) functions were completed to when the I/O's + completion was reaped by fio. **lat** Total latency. Same names as slat and clat, this denotes the time from when fio created the I/O unit to completion of the I/O operation. + It is the sum of submission and completion latency. **bw** Bandwidth statistics based on samples. Same names as the xlat stats, -- 2.25.1
