varnishtop stdin

[Michael Iams]

We are seeing an issue where one-for-one increments in "sms_nreq/SMS allocator requests" are resulting in 503 errors.   We are not getting any backend related errors.
This is happening on a very small percentage of the page requests, but it is repeatable in our load-tests.

A couple questions:
   1)  Is this a problem that has been seen before?  Is there a known configuration adjustment, work-around?
   2)  What specific function is performed by the SMS metrics measured in varnishstat?   The most I've found in my digging is that it is used for internal memory house-keeping.  There was a similar inquiry a couple of days ago on this message-list regarding SMS, SMA, SM, etc.   It would be good if there was something published somewhere that elaborates on what these measure.