Clarify spread/lambda of poisson

[fio.git] / HOWTO

diff --git a/HOWTO b/HOWTO
index d3ead19782796af8dc5b0b496a65318cc3bcf210..a534aa865d8c4a9384ebee3d823ecaaaf0fdfb3d 100644 (file)
--- a/HOWTO
+++ b/HOWTO
@@ -812,8 +812,10 @@ iodepth_batch_submit=int
 iodepth_batch=int This defines how many pieces of IO to submit at once.
                It defaults to 1 which means that we submit each IO
                as soon as it is available, but can be raised to submit
-               bigger batches of IO at the time.
+               bigger batches of IO at the time. If it is set to 0 the iodepth
+               value will be used.
 
+iodepth_batch_complete_min=int
 iodepth_batch_complete=int This defines how many pieces of IO to retrieve
                at once. It defaults to 1 which means that we'll ask
                for a minimum of 1 IO in the retrieval process from
@@ -823,6 +825,31 @@ iodepth_batch_complete=int This defines how many pieces of IO to retrieve
                events before queuing more IO. This helps reduce
                IO latency, at the cost of more retrieval system calls.
 
+iodepth_batch_complete_max=int This defines maximum pieces of IO to
+               retrieve at once. This variable should be used along with
+               iodepth_batch_complete_min=int variable, specifying the range
+               of min and max amount of IO which should be retrieved. By default
+               it is equal to iodepth_batch_complete_min value.
+
+               Example #1:
+
+               iodepth_batch_complete_min=1
+               iodepth_batch_complete_max=<iodepth>
+
+               which means that we will retrieve at leat 1 IO and up to the
+               whole submitted queue depth. If none of IO has been completed
+               yet, we will wait.
+
+               Example #2:
+
+               iodepth_batch_complete_min=0
+               iodepth_batch_complete_max=<iodepth>
+
+               which means that we can retrieve up to the whole submitted
+               queue depth, but if none of IO has been completed yet, we will
+               NOT wait and immediately exit the system call. In this example
+               we simply do polling.
+
 iodepth_low=int        The low water mark indicating when to start filling
                the queue again. Defaults to the same as iodepth, meaning
                that fio will attempt to keep the queue full at all times.
@@ -1022,7 +1049,7 @@ rate=int  Cap the bandwidth used by this job. The number is in bytes/sec,
                will only limit writes (to 500KB/sec), the latter will only
                limit reads.
 
-ratemin=int    Tell fio to do whatever it can to maintain at least this
+rate_min=int   Tell fio to do whatever it can to maintain at least this
                bandwidth. Failing to meet this requirement, will cause
                the job to exit. The same format as rate is used for
                read vs write separation.
@@ -1037,6 +1064,15 @@ rate_iops_min=int If fio doesn't meet this rate of IO, it will cause
                the job to exit. The same format as rate is used for read vs
                write separation.
 
+rate_process=str       This option controls how fio manages rated IO
+               submissions. The default is 'linear', which submits IO in a
+               linear fashion with fixed delays between IOs that gets
+               adjusted based on IO completion rates. If this is set to
+               'poisson', fio will submit IO based on a more real world
+               random request flow, known as the Poisson process
+               (https://en.wikipedia.org/wiki/Poisson_process). The lambda
+               will be 10^6 / IOPS for the given workload.
+
 latency_target=int     If set, fio will attempt to find the max performance
                point that the given workload will run at while maintaining a
                latency below this target. The values is given in microseconds.
@@ -1054,7 +1090,7 @@ latency_percentile=float  The percentage of IOs that must fall within the
 max_latency=int        If set, fio will exit the job if it exceeds this maximum
                latency. It will exit with an ETIME error.
 
-ratecycle=int  Average bandwidth for 'rate' and 'ratemin' over this number
+rate_cycle=int Average bandwidth for 'rate' and 'rate_min' over this number
                of milliseconds.
 
 cpumask=int    Set the CPU affinity of this job. The parameter given is a
@@ -1150,6 +1186,9 @@ mem=str           Fio can use various types of memory as the io unit buffer.
                                backing. Append filename after mmaphuge, ala
                                mem=mmaphuge:/hugetlbfs/file
 
+                       mmapshared      Same as mmap, but use a MMAP_SHARED
+                               mapping.
+
                The area allocated is a function of the maximum allowed
                bs size for the job, multiplied by the io depth given. Note
                that for shmhuge and mmaphuge to work, the system must have