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тАО07-29-2001 05:23 PM
тАО07-29-2001 05:23 PM
Performance impact of Pseudo Swap
I have heard and read that enabling Pseduo Swap (swapmem_on kernel parameter) degrades system performance. Some documentation I have seen recommend to turmn it off for optimal performance.
I know what Pseudo Swap does and its impact on lockable memory but I do not know how it might impact or degrades performance if it is turned on.
Can any of you HP-UX performance specialists out there let me know your thoughts on this.
Thanks,
Trevor
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тАО07-29-2001 05:58 PM
тАО07-29-2001 05:58 PM
Re: Performance impact of Pseudo Swap
Pseudo swap allows more processes to run than swap devices can accomodate. Pseudo swap space does not exist in reality, rather, arming it only inflates the calculation of device and file swap space.
Beginning with HP-UX 10.0 memory management algorithms were rewritten. Instead of "swapping" in and out whole processes, pages (4K) of a process are now "paged" in and out as necessary. Thus, when whole processes had to be moved in and out of memory, one could argue that declaring pseudo swap would allow more processes to run but at the expense of large amounts of I/O.
With paging occuring in much smaller granules, and only after inuse memory has "aged", it is probable that memory can be found for a new process (or to resume an already running one) with significantly less I/O expense than whole process swapping.
Given a system with an adequate memory configuration, "swapping" (or "paging" as is really should be called) should be very low in the first place, and the issue mote.
...JRF...
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тАО07-29-2001 06:41 PM
тАО07-29-2001 06:41 PM
Re: Performance impact of Pseudo Swap
I guess I can make the question more specific as follows
On a system with sufficient RAM to ensure that no paging to/from swap is occuring, does enabling pseudo swap degrade system performance in any way. If so then why.
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тАО07-30-2001 03:30 AM
тАО07-30-2001 03:30 AM
Re: Performance impact of Pseudo Swap
pseudo-swap is a powerfull feature if you need it, and a worest one if you don't need it.
If pseudo-swap is on that's means, your dbc_max_pct " Dynamic buffer cache max percentage " kernel parameter will be taken into account :
hp-ux will anticipate reading data from disk into memory to make it available to process, this process allow process to be almost running and to avoid the state "w" which is waiting for I/Os. But if you do not use these data in the memory that means that your are reading disk, allocating memory for nothing.
Attention : by default the dbc_max_pct is set to 50 % of the entire memory -> So, each time you are reading and allocating 50 % of memory for nothing.
Conclusion :
It's a good think if you handle huge flat files and a bad one if you handle a database.
Magdi
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тАО07-30-2001 03:49 AM
тАО07-30-2001 03:49 AM
Re: Performance impact of Pseudo Swap
First I only set up device swap as (non-tech talk here) a last resort for my systems to swap to, but I don't want them to use it...it's just there as last resort and dump space to me.
I tend to turn pseudo_swap ON. Why? Because I want my system to process in memory - it's faster! So I guess to those folks who say turn it off...I would say ....why? Why would you allow your system to fall back to the degradation of device I/O when you could run faster and more efficiently in memory. That is why you buy more memory....to run everything faster. So why would you circumvent the system's ability to run the jobs in memory by turning pseudo_swap off....?
Does pseudo_swap degrade the system? I feel it doesn't-it simply makes the system WORK to it's peak performance. Why have memory, if your not going to make it work for you?
I love illustrations...it's like wanting eggs for breakfast..you go out and buy the eggs, you put them in a pan...but you never let them cook. Turn it on and let your system start doing the cookin'..
Just my thoughts,
Rita
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тАО07-30-2001 04:02 AM
тАО07-30-2001 04:02 AM
Re: Performance impact of Pseudo Swap
Pseudo swap has no negative performance impact.
Pseudo swap has been invented for a simple reason. On systems with sufficient memory it is not really necessary to reserve swap space for every process, as it is pretty unlikely that we ever start paging. And when we don't need to page, why should we reserve the space needed for it?? With the pseudo swap parameter enabled we are simply NOT reserving swap space for a process as long as the memory utilization is below 75%. You see that we really do less than before. Enabling pseudo swap pretty much behaves as if you have added a swap device with a capacity of 75% of your physical memory.
If more than 75% memory is used, we use the normal algorithm again and reserve swap space for every new process, as it now becomes more likely that we need to page out processes.
For the vast majority of systems it makes sense to enable pseudo swap.
Carsten
In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move. -- HhGttG
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тАО07-30-2001 04:39 AM
тАО07-30-2001 04:39 AM
Re: Performance impact of Pseudo Swap
I have reviewed again the document that prompted me to ask the question. The document states that if device swap is at least equal to physical memory and that all of physical memory is never likely to be completely used then to disable pseudo swap as it causes additional system overhead even if virtual memory paging never occurs (ie available memory never drops below the desfree kernel parameter). Unfortunately due to non-disclosure agreement I can not disclose further content or the origin of the document.
Thanks everyone for your advice, I will try to be clearer when asking questions in future.
-Trevor
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тАО07-30-2001 04:41 AM
тАО07-30-2001 04:41 AM
Re: Performance impact of Pseudo Swap
/usr/share/doc/mem_mgt.txt
and search for "Pseudo-Swap Space".
Carsten
In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move. -- HhGttG
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тАО07-30-2001 04:53 AM
тАО07-30-2001 04:53 AM
Re: Performance impact of Pseudo Swap
The performance impact of Pseudo swap can only happen if you actually run out of device swap. So if device swap is < 100% reserved then it has no impact at all.
If you, for example, have device swap of 1GB and RAM=1GB and your using the entire 1 GB of RAM (no free mem) then pseudo swap can now be used and its performance impact would be very heavy due to > 1GB of programs trying to use only 1 GB of RAM.
Pseudo-swap is only really a safety net. If it was off and you run of device swap then programs will die not being able to allocate space, whereas if pseudo-swap was on at least they would run - albeit slowly and at the cost of your server slowing down a bit.
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тАО07-30-2001 05:08 AM
тАО07-30-2001 05:08 AM
Re: Performance impact of Pseudo Swap
Carsten's post comes closest to answering my question in his reply:
"Pseudo swap has no negative performance impact."
..although this statement contradicts the document I have seen.
Please do not post any further explanations of how pseudo swap/virtual memory paging works as I already have intimate knowledge of this (including the memory management white paper).
I will drop this until I can speak with someone who has knowledge of how pseudo swap is handled internally within the kernel.
I appreciate the effort everyone has put into providing answers but please consider the thread closed.
Regards,
Trevor