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Introduction to Memory Bottlenecks
If ever your Windows 2003 server is running slowly, then the first place to look for a bottleneck
is memory. Another way of looking at server performance, is that machines with
plenty of RAM rarely give problems. A bonus of plenty of memory is,
that to a degree, abundant RAM compensates for
strain on other resources.
On old servers, lack of memory would give you the full sensory input, you could hear the disk paging, see the light flashing, and 'Mad' Mick swears you could smell the disk thrashing. Even with these
sensory clues, it is still worth while monitoring memory with Performance Logs. Please also remember the big picture. So once you have had a quick look at memory, remember to check the processor and
disk counters.
The servers most likely to suffer from memory shortage are pure database servers
for example, Oracle or SQL. Email server also require plenty of RAM.
Pure domain controllers are less likely to experience memory problems.
Memory Topics
The more available memory the faster the server can respond. When I check a server's memory with performance monitor, the first
counter that I add to the log is Memory\Available bytes. As long as the trace
indicates more than 10MB of free memory, then I conclude that the server has
sufficient RAM.
Diagram 1 shows a white descending line,
and the legend confirms
that Available bytes are down to 3MB. Clearly this machine needs more memory.
Suppose a spreadsheet wants to start a new thread or a database needs to
sort data, what each needs is memory. The operating system
provides this memory at
least 100 times faster using RAM, than it could using a disk based pagefile. This is why a
large pool of free memory is so important to an application server.
Take care to distinguish between these two paging counters:
1) Pages
/sec (Hard page faults)
2) Page Faults /sec counter is likely to be at
least twice the value of the above.
Two problems with monitoring in general, firstly no counter should
be taken in isolation, secondly spikes should be ignored, or at least played
down. The less paging the better your server's performance. Most authorities
agree that Memory: Pages / sec is a key memory counter. This counter
measures 'hard' page faults, in other words the page in nowhere in memory,
so the VMM (Virtual Memory Manager) has to fetch the data from the pagefile
on the disk; in computing terms that takes an age.
I am reluctant to disagree with other authorities, but from my experience,
I would put the threshold as high as 20 pages /sec, before blaming paging as
the bottleneck. Moreover, I
would not trust pages /sec as an indicator of a bottleneck without confirmation from low Available bytes.
(see above)
In truth, if you put 5 experts in the same room, they could all spot a
memory bottleneck, but when they wrote up their notes, they would use
different time slices and different thresholds, consequently, it would seem that there
was a conflict where none actually existed.
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