AIX 5L Problem Determination - IBM Redbooks

The sr column
The sr column shows the number of pages that were examined per second by
the page-replacement algorithm during the interval. The VMM page-replacement
code scans the PFT and steals pages until the number of frames on the free list
is at least the maxfree value. The page-replacement code might have to scan
many entries in the PFT before it can steal enough to satisfy the free list
requirements. With stable, unfragmented memory, the scan rate and free rate
might be nearly equal. On systems with multiple processes using many different
pages, the pages are more volatile and disjointed. In this scenario, the scan rate
might greatly exceed the free rate.
Memory is over committed when the ratio of fr to sr (fr:sr) is high. A fr:sr ratio of
1:4 means that for every page freed, four pages had to be examined. It is difficult
to determine a memory constraint based on this ratio alone, and what constitutes
a high ratio is workload/application dependent.
The cy column
The cy column shows the number of cycles per second of the clock algorithm.
The VMM uses a technique known as the clock algorithm to select pages to be
replaced. This technique takes advantage of a referenced bit for each page as an
indication of what pages have been recently used (referenced). When the
page-stealer routine is called, it cycles through the PFT, examining each page's
referenced bit. The cy column shows how many times per second the
page-replacement code has scanned the PFT. Because the free list can be
replenished without a complete scan of the PFT, and because all of the vmstat
command fields are reported as integers, this field is usually zero. If not, it
indicates a complete scan of the PFT, and the stealer has to scan the PFT again,
because fre is still under the maxfree value.
One way to determine the appropriate amount of RAM for a system is to look at
the largest value for avm reported by the vmstat command. Multiply that by 4 KB
to get the number of bytes, and then compare that to the number of bytes of RAM
on the system. Ideally, avm should be smaller than the total RAM. If not, some
amount of virtual memory paging will occur. How much paging occurs will
depend on the difference between the two values. Remember, the idea of virtual
memory is that it gives us the capability of addressing more memory than we
have (some of the memory is in RAM and the rest is in paging space). If there is
far more virtual memory than real memory, this could cause excessive paging,
which then results in delays. If avm is lower than RAM, then paging-space
paging could be caused by RAM being filled up with file pages. In that case,
tuning the minperm or maxperm values could reduce the amount of
paging-space paging. This can be done with the vmtune command.
Another useful command for memory performance problem determination is the
ps command.
250 IBM ^ Certification Study Guide - AIX 5L Problem Determination Tools and Techniques