:Has anyone heard of performance degradation caused by having both
:VAX 6000s and DEC 7000s on the same CI bus? I was talking with
:another system dude and he mentioned that his cluster (which has both
:VAX 6000 and Alpha 7000 systems) runs "slower" than if he only had the
:7000s and the HSJ42s on that CI.
You might want to ask what this other "system dude" was refering to.
I can only guess what he meant. :-)
:I've never heard of a VAX 6000 "slowing down" the CI bus? Is this true?
There are VAX 6000 systems ranging from excruitatingly slow to just
plain slow -- example systems include the 2.8 VUPS or so for the VAX
6000-210, and 13.0 VUPS or so for the VAX 6000-510.
:If so, please elaborate on why the 6000s slow things down.
Slower systems tend to hold onto locks and tend to block "critical
paths" longer -- I ran one of the classic "odd couple" VMScluster
configurations in years past, a VAX-11/750 (circa 0.6 VUPs) and a
VAX 8800 (circa 6 VUPS per CPU). When the VAX-11/750 accessed a
common file or a common data structure, particularly something that
the VAX 8800 needed to access, the VAX 8800 would sit-and-spinlock
until the slower VAX-11/750 (slowly) finished with it. One could
notice when the VAX-11/750 started heavily accessing (and locking)
SYSUAF, for instance.
This behaviour will obviously become more noticable among systems with
shared resources and rather larger performance disparities, and when a
faster system waits for a lock served by or held by the slower system.
I would tend to use the LOCKDIRWT SYSGEN parameter to "encourage" locks
and resource mastership to occur on the faster node(s)...
As for "slowing down the CI", the mere presense of another CI adapter
on the CI has little or no impact on CI performance.
Discussions of the (very small) overhead involved with polling additional
CI adapters will be ignored, as will all discussions of the dual-path vs
single-path CI adapters, as I would guess that neither of these were what
the "system dude" was refering to. :-)
-------------------------- pure personal opinion ---------------------------
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