Monitor CPU level

[Volker Halle]

smsc,

a high BUFIO rate just indicates lots of buffered IOs (e.g. DECnet, LAT or TCPIP traffic or File System IOs) and is not 'dangerous'. The value 500 is not some kind of maximum, it is just an arbitrary value choosen by MONITOR as the x-axis maximum value in the display.

Merry Christmas,

Volker.

[Hoff]

What's your goal here? System-level issues? Performance of a specific application? Confirming what information your support folks are providing? Looking for a job tuning applications or operating systems?

400 or 500 buffered I/Os could be anything from operating at little more than an idle to a full-blown performance crisis, depending on application requirements and application timing.

Buffered I/O and Direct I/O differ in the use of a buffer allocated from within a pool of memory maintained by OpenVMS. Buffered I/O involves memory copies, while Direct I/O involves setting up DMA transfers. The former are for slower devices, the latter are for devices that offer DMA. In practical terms, they're all I/O operations. And some of these are slow, and some I/O operations are fast.

There's no simple answer to a performance question, and there's no chance that isolated monitoring will produce a meaningful result. I've found "hit-and-run tuning" can lead to issues with system performance; if you optimize for an unusual or boundary case, you might not get the intended benefits. Or you might miss a big win.

The answer usually arises with the trends and in the averages, and in factors such as the application response timing.

Consider: is the 400 or 500 BIO count really a spike, a trough, or an average? Does it tie in with the observed performance? What's the trend?

And as for application-level issues, I've regularly been surprised at the performance-limiting factor(s) within a typical non-trivial application. I might think "I/O" and end up looking at a one-page compute-bound critical loop buried in some corner of the code. DECset PCA, the SDA PC monitoring tools, and other such instruments are invaluable here.

Go systematic and go wide with your system monitoring. Go TDC and T4. Don't enter into the whole of the tuning discussion with preconceptions or assumptions. Monitor it all, and zero in from there. And work with your system staff here.