[zeromq-dev] Getting rid of low watermark

[Martin Sustrik]

Hi all,

Currently, when setting limit for number of messages that can be held by 
  a socket you have to specify high & low watermarks.

Here's what documentation says:

"The ZMQ_HWM option shall set the high water mark for the message queue 
associated with the socket. The high water mark is a hard limit on the 
number of outstanding messages in the queue; if this
        limit has been reached the socket shall enter an "emergency" 
state and depending on the socket type, 0MQ shall take appropriate 
action such as blocking or dropping new messages entering the
        queue."

"The ZMQ_LWM option shall set the low water mark for the message queue 
associated with the socket. This option only makes sense when used in 
conjunction with the ZMQ_HWM option. A socket which
        has reached it’s high water mark remains in the "emergency" 
state until the number of outstanding messages in it’s associated 
message queue falls below the low water mark, at which point normal
        message processing is resumed."

As far as I understand -- correct me if I am wrong -- it's not obvious 
what value LWM should be set to and it's not even clear why the user 
should bother with setting LWM at all.

Given the above, I would like to propose an algorithm for automatic 
setting of LWM.

Following three points are to be considered:

1. LWM has to be less than HWM.

2. LWM cannot be set to very low value (such as zero) as after filling 
the queue it would start to refill only after all the messages are read 
from it and thus unnecessarily hold the progress back.

3. LWM cannot be set to very high value (such as HWM-1) as it would 
result in lock-step filling of the queue -- if a single message is read 
from a full queue, writer thread is resumed to write exactly one message 
to the queue and go back to sleep immediately. This would result in low 
performance.

Given the 3) it would be good to keep HWM and LWM as far apart as to 
reduce the thread switching overhead to almost zero, say HWM-LWM should 
be 500.

Still we have to account for the cases where HWM<500 thus driving LWM to 
negative numbers. My suggestion is to make LWM one half of HWM in such 
cases.

The graph showing resulting HWM/LWMs is attached.

Thoughts?
Martin

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