Token-based flow control

Token-based flow control

This patch allows two fio jobs to be kept to a certain
proportion of each other using token-based flow control.
There are three new parameters: flow, flow_watermark, and
flow_sleep, documented in the fio options. An example of an fio
job using these parameters is below:

[global]
norandommap
thread
time_based
runtime=30
direct=1
ioengine=libaio
iodepth=256
size=100g
bs=8k
filename=/tmp/testfile
flow_watermark=100
flow_sleep=1000

[job2]
numjobs=1
rw=write
flow=-8

[job1]
numjobs=1
rw=randread
flow=1

The motivating application of this patch was to allow random reads
and sequential writes at a particular given proportion.

This initial version is only correct when run with 'thread', as shared
state is represented with a global variable. It also only allows two
jobs to be synchronized properly. A future version might do more, but
no more functionality was needed for my application.

Tested: Ran a few fio jobs with this flow control, observing
the proportion of IOPS to match what was intended by the job file.
Varied the flow_watermark and flow_sleep parameters and observed
the effect on throughput.

Signed-off-by: Dan Ehrenberg <dehrenberg@google.com>
Modified by me to support flow_id, so an arbitrary number of flows can
be used. This means it no longer relies on global context, so it can be
used from a thread or process alike. Also added man page documentation.

Signed-off-by: Jens Axboe <axboe@kernel.dk>