| Overview |  |
Behaviours: sbroker_fair_queue, sbroker_queue.
References
Implements a head or tail drop queue with queue management based on CoDel (Controlling Queue Delay).
sbroker_codel_queue can be used as a sbroker_queue in a sbroker or
sregulator. It will provide a FIFO or LIFO queue that drops request in the
queue using CoDel when the minimum size is exceeded, and drops the head or
tail request from the queue when a maximum size is exceeded. Its argument,
spec(), is of the form:
#{out => Out :: out | out_r, % default: out
target => Target :: non_neg_integer(), % default: 100
interval => Interval :: pos_integer(), % default: 1000
drop => Drop :: drop | drop_r, % default: drop_r
min => Min :: non_neg_integer(), % default: 0
max => Max :: non_neg_integer() | infinity} % default: infinity
Out is either out for a FIFO queue (the default) or out_r for a LIFO
queue. Target is the target queue sojourn time in milliseconds (defaults to
100). Interval is in the initial interval in milliseconds (defaults to
1000) between drops when the queue is above the minimum size
Min (defaults to 0). Drop is either drop_r for tail drop (the
default) where the last request is droppped, or drop for head drop, where
the first request is dropped. Dropping occurs when queue is above the
maximum size Max (defaults to infinity).
Initial parameters are recommended to be between the 95th and 99th percentile
round trip time for the interval and the target between 5% and 10% of the
interval. The round trip should be that between the actual initiator of the
request (e.g. a remote client) and the queue. For example, the reference
suggests an interval of 100ms and a target of 5ms when queuing TCP packets in
a kernel's buffer. A request and response might be a few more round trips at
the packet level even if using a single :gen_tcp.send/2.
A person perceives a response time of 100 milliseconds or less as
instantaneous, and feels engaged with a system if response times are 1000
milliseconds or less. Therefore it is desirable for a system to respond
within 1000 milliseconds as a worst case (upper percentile response time)
and ideally to respond within 100 milliseconds (target response time).
These also match with the suggested ratios for CoDel and so the default
target is 100 milliseconds and the default is interval 1000 milliseconds.
This implementation differs from the reference as enqueue and other functions
can detect a slow queue and drop items. However once a slow item has been
detected only handle_out/2 can detect the queue becoming fast again with
the caveat that with out_r this can only be detected if the queue reaches
the minimum size Min or less, and not a fast sojourn time. This means that
it is possible to drop items without calling handle_out/2 but it is not
possible to stop dropping unless a handle_out/2 dequeues an item to take
the queue to or below the minimum size Min or out is used and a dequeued
request is below target sojourn time. Therefore if handle_out/2 is not
called for an extended period the queue will converge to dropping all items
above the target sojourn time if a single item has a sojourn time above
target. Whereas with the reference implementation no items would be dropped.
Min above 0 can leave Min items in the
queue for an extended period of time as requests are only dropped when the
queue size is above Min. This may be undesirable for client requests
because the request could wait in the queue indefinitely if there are not
enough requests to take the queue above Min. However it might be desired
for database connections where it is ideal for a small number of connections
to be waiting to handle a client request.
spec() = #{out => Out::out | out_r, target => Target::non_neg_integer(), interval => Interval::pos_integer(), drop => Drop::drop | drop_r, min => Min::non_neg_integer(), max => Max::non_neg_integer() | infinity}
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