| Oracle FAQ | Your Portal to the Oracle Knowledge Grid | |
 |
 | |||
| Oracle FAQ | Your Portal to the Oracle Knowledge Grid | |
|
| |||
Home -> Community -> Mailing Lists -> Oracle-L -> Re: OT: question about sizing swap for solaris
It makes no sense to me. An analogous statement would be "Buying a large gas
tank may prevent you from realizing what a gas-guzzler you are driving." At the
point where you use so much swap that physical ("hard") page-outs to swap space
(or, more accurately, the hard page-ins that could follow) would become a
problem, the alternative, if you had sized swap more modestly, would be
out-of-memory errors (like running out of gas). The only possible advantage of
that alternative is that it is harder to ignore out-of-memory errors than to
ignore poor performance attributable to excessive paging. The real problem (in
theory), whether you get poor performance, or prevent it by triggering even
worse errors by making swap run out before poor performance sets in, is that
you don't have enough memory to run the application without excessive
paging/swapping.
That's the theory. In practice, memory is *cheap* and there is virtually always *plenty* of it, so if you are getting out-of-memory errors, you are probably nowhere near having paging problem and should just increase swap (which can often be justified to be as much as 5 times the size of real memory, *if memory wasn't over-sized* (not that over-sizing memory is so bad - it *is* cheap)). (The average real-world application has an immense amount of virtual memory dedicated to very idle processes and very idle pages of processes, such that paging and swapping out these idle parts of virtual memory bears almost no cost to performance at all, contrary to popular belief. I have seen busy produciton systems with fully half their processes swapped out, and much of the remainder paged out, with very low actual activity on the swap disks, translating to very little actual performance impact on the end users.)
Q: In almost 15 years of work on performance, how many times have I seen a system with paging and swapping performance costs that were high, without a special cause such as a bug in the swapping algorithm?
Dan Tow
650-858-1557
www.singingsql.com
Quoting zhu chao <chao_ping_at_vip.163.com>:
> Hi,
> I am reading sun performance paper: Performance Oriented System
> Administration and see the following setense:
> "However, sizing swap to be too large can cause hard-pageout to occur too
> easily"
> I do not understand what on earth this mean, can some one kindly explain
> this? And why it will make hard-pageout occur?Is there documents talking
> about this feature?
> Thanks.
> Zhu Chao.
-- Archives are at http://www.freelists.org/archives/oracle-l/ FAQ is at http://www.freelists.org/help/fom-serve/cache/1.html -----------------------------------------------------------------Received on Mon May 03 2004 - 15:01:19 CDT
 |
 |