Sybase 12.4.2 Monitoring paging on Windows NT systems, Monitoring paging on Unix systems, 477

CHAPTER 13 Monitoring and Tuning Performance

Buffer manager thrashing occurs when the operating system chooses less optimum buffers to page out to disk, which forces the buffer manager to make extra reads from disk to bring those buffers back to memory. Since Adaptive Server IQ knows which buffers are the best candidates to flush out to disk, you want to avoid this operating system interference by reducing the overall number of page outs.

When you set buffer sizes, keep in mind the following trade-off:

•If the IQ buffer cache is too large, the operating system is forced to page as Adaptive Server IQ tries to use all of that memory.

•If the IQ buffer cache is too small, then Adaptive Server IQ thrashes because it cannot fit enough of the query data into the cache.

If you are experiencing dramatic performance problems, you should monitor paging to determine if thrashing is a problem. If so, then reset your buffer sizes as described in “Managing buffer caches”.

Monitoring paging on Windows NT systems

Windows NT provides the NT Performance Monitor to help you monitor paging. To access it, select the object Logical Disk, the instance of the disk containing the file PAGEFILE.SYS, and the counter Disk Transfers/Sec. This should be on a separate disk from your database files. You can also monitor the Object Memory and the counter Pages/Sec. However, this value is the sum of all memory faults which includes both soft and hard faults.

Monitoring paging on UNIX systems

UNIX provides a system command, vmstat, to help you monitor system activity such as paging. The abbreviated command syntax is:

vmstat

interval count

The interval is the time between rows of output, and count is the number times a row of output is displayed. For more information about vmstat (including its options and field descriptions), see your operating system's documentation.

Here is an example:

>vmstat 2 3

procs	memory		page	disk	faults	cpu
r b w	swap	free	re mf pi po	fr de sr s0 s1 sd	in sy	cs us sy id

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