Virtual Memory Settings?

Virtual Memory Settings?

Post by Joshua McAdam » Tue, 08 Jul 2003 06:58:19



I'm an end user working on a box with a fair amount of memory and a really
low virtual memory setting in comparision to RAM: 4096.0MB VM to 24575.8MB
RAM.  Anyway, the box isn't remarkably consistant in performance from week
to week so I'm trying to help troubleshoot what is wrong.  In all of the
docs that I've been looking over, the common sentement seems to be to have
your virtual memory set to at least 1.2xRAM.  We are nowhere near that, but
from looking at the nmon output below (during a fairly busy point in our
update), we don't even touch the virtual memory that we have (1%).

My thoughts are that I'm barking up the wrong tree and need to move on...
would you agree?

Also, the 'DANGER' line of the Verbose Mode output is a little disturbing,
but I'm not sure what I'm looking at there... other than a really busy
disk(s).  Our CPU wait is at 0, so I don't know if this is affecting us at
all.

Any help would be greatly appreciated.

Thanks in advance,
Josh

nmon v8b [H for help]  Hostname= Refresh=2.0secs  16:50.51
Verbose Mode
 Code    Resource            Stats   Now        Warn    Danger
     OK -> CPU               %busy  19.7%       >80%    >90%
Warning -> Paging Space      %free  99.0%       VM<RAM  <20%
Warning -> Page Faults       faults 55.0        >32/s   >320/s
 DANGER -> Top Disk  hdisk22 %busy  98.4%       >40%    >60%

CPU Utilisation
+-------------------------------------------------+
CPU    User%  Sys% Wait% Idle|0          |25         |50          |75
100|
 0     22.5   0.0   0.0  77.5|UUUUUUUUUUU              >
|
 1     33.0   0.0   0.0  67.0|UUUUUUUUUUUUUUUU>
|
 2      7.5   0.0   0.0  92.5|UUU                        >
|
 3     55.2   0.0   0.0  44.8|UUUUUUUUUUUUUUUUUUUUUUUUUUU>
|
 4      9.5   0.0   0.0  90.5|UUUU                         >
|
 5      2.5   1.0   0.0  96.5|U               >
|
 6      5.5   2.0   0.0  92.5|UUs                   >
|
 7     12.5   1.0   0.0  86.5|UUUUUU>
|
 8     60.5   1.0   0.0  38.5|UUUUUUUUUUUUUUUUUUUUUUUUUUUUUU         >
|
 9      6.5   0.0   0.0  93.5|UUU   >
|
10      0.0   2.5   0.5  97.0|s                                          >
|
11      9.5   0.5   0.0  90.0|UUUU        >
|
12     43.0   0.0   0.0  57.0|UUUUUUUUUUUUUUUUUUUUU>
|
13     20.5   0.5   0.0  79.0|UUUUUUUUUU>
|
14      3.5   3.5   0.0  93.0|Us     >
|
15     11.5   0.5   0.0  88.0|UUUUU>
|

+-------------------------------------------------+
       18.9   0.8   0.0  80.3|UUUUUUUUU>
|

+-------------------------------------------------+
Memory Use  Physical    Virtual    Paging pages/sec  In     Out  VM
parameters
% Used        69.2%       1.0%      to Paging Space   0.0    0.0 numperm
50.8%
% Free        30.8%      99.0%      to File System    0.0   13.0 minperm
4.7%
MB Used    17005.1MB     40.4MB    Page Scans         0.0        maxperm
9.5%
MB Free     7570.8MB   4055.6MB    Page Cycles        0.0        minfree
1024
Total(MB)  24575.8MB   4096.0MB    Page Reclaim       0.0        maxfree
1280

DiskName Busy  Read  Write   |0          |25         |50          |75
100|
hdisk0     0%    0.0    0.0KB|                                  >
|
hdisk3     0%    0.0    0.0KB|>
|
hdisk2     0%    0.0    0.0KB|>
|
hdisk1     0%    0.0    0.0KB|                                            >
|
cd0        0%    0.0    0.0KB|>
|
cd1        0%    0.0    0.0KB|>
|
dac0       0%    0.0    8.0KB|>
|
dac1       0%    0.0    8.0KB|>
|
dac2       0%    0.0    0.0KB|>
|
dac3       0% 9032.3 6202.7KB|>
|
dac4       0%    0.0    8.0KB|>
|
dac5       0%    0.0    0.0KB|>
|
dac6       0%    0.0   24.0KB|>
|
dac7       0%    0.0 1055.1KB|>
|
dac8       0% 8105.1 10319.2K|>
|
dac9       0% 1518.7 1510.7KB|>
|
hdisk4     0%    0.0    8.0KB|
|
hdisk5     0%    0.0    8.0KB|     >
|
hdisk6     0%    0.0    0.0KB|
|
hdisk7     0%    0.0    0.0KB|                   >
|
hdisk8     0%    0.0    0.0KB|
|
hdisk9     0%    0.0    0.0KB|
|
hdisk10    0%    0.0    0.0KB|   >
|
hdisk11    0%    0.0    8.0KB|     >
|
hdisk12    0%    0.0    0.0KB|>
|
hdisk13    0%    0.0    0.0KB|
|
hdisk14    0%    0.0   24.0KB|
|
hdisk15    0%    0.0    0.0KB|  >
|
hdisk16    0%    0.0    0.0KB|     >
|
hdisk17    0%    0.0    0.0KB|  >
|
hdisk18   55% 3804.8    0.0KB|RRRRRRRRRRRRRRRRRRRRRRRRRRRR
|RRRRRRRRRRRRRRRRRRRRRRRRRRR
hdisk19    6%    0.0 1079.1KB|WWWW
|
hdisk20   26% 5227.6 6202.7KB|WWWWWWWWRRRRRR
|RR
hdisk21    0%    0.0    0.0KB|
|
hdisk22   99% 2581.8
4436.2KB|WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWRRRRRRRRRRRRRRRRR|RWWWWWWWWWWWWWWWW
WWWWWWWWWWWWWWWWWWWWWWWWWWWW
hdisk23   25% 1518.7 1510.7KB|WWWWWWWRRRRRRR
|WWWRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR
hdisk24   31% 5523.3 5883.0KB|WWWWWWWWRRRRRRRR
|RRRRR>RRRRRRRRRRRRRRRRRRRR
hdisk25    0%    0.0    0.0KB|>
|

 
 
 

Virtual Memory Settings?

Post by Dan Foste » Tue, 08 Jul 2003 07:19:57



Quote:> I'm an end user working on a box with a fair amount of memory and a really
> low virtual memory setting in comparision to RAM: 4096.0MB VM to 24575.8MB
> RAM.  Anyway, the box isn't remarkably consistant in performance from week
> to week so I'm trying to help troubleshoot what is wrong.  In all of the
> docs that I've been looking over, the common sentement seems to be to have
> your virtual memory set to at least 1.2xRAM.  We are nowhere near that, but

Ignore the "set virtual memory to..." stuff -- you just size it to match
your expected workload. From the numbers you posted, virtual memory is
*NOT* at all a problem... just about none of it is being used at all.
(40MB out of 17,000MB of virtual memory committed) This is what I would
expect from a well written application on a large system that uses its
memory as (essentially) one large cache to offset the cost of doing disk
I/O. (Disk I/O is generally at least 1,000 times slower than memory I/O!)

There's some CPU loading from your application but it's not severe, and
there's usually 40-60% idle time.

So all perf issues usually comes from one of these three areas:

CPU usage, memory usage, I/O performance.

CPU looks fine, memory looks fine. Your I/O looks really stressed -- I see
some hot spots... such as hdisk22 where the I/O wait is 99%+, and a few
other disks with a significant enough I/O wait.

It isn't so much a case of "Getting faster disks" -- hdisk22 sounds like a
possible RAID or volume set of some sort... it's more a case of spreading
out the I/Os across more spindles (disks) rather than slamming only one
physical volume (disk) so heavily.

There are 'features' of AIX where traditionally, all heavy I/Os done to a
volume group for the single jfs (journalled filesystem) log was done on a
single disk because that's how it's configured by default... which becomes
an hot spot if you have a *lot* of small filesystem changes to the point
where the single disk holding the jfs log can't keep up with all the I/O
transactions in a timely manner. (AIX 5.2 finally fixes this better.)

To fix that, may be a possible combination of retooling application a bit
better to spread out the I/O, as well as rethinking the underlying disk
layout strategy -- maybe add a fast-write RAID cache to the controller or
to break it out to some sort of non-RAID-5 array, or whatever.

With serious I/O wait, aka 'hot spots', it is possible for the entire
performance of the application to come to a screeching halt while it waits
for data to be returned from disk (or for disk to complete a write).

So I bet this is your performance issue. You just need to look at the
actual numbers you posted and think about what it's trying to tell you,
rather than trying to fudge a predetermined conclusion. So... :)

(The above paragraph was meant for your system admin. I admire you having
taken the initiative to research this issue.)

-Dan

(mailed to original poster and posted to USENET news, as a courtesy.)

 
 
 

1. setting of virtual memory

We have a RS6000 with Unix AIX 4.3 and have
some problem with memory management.

We have 192MB (we came from 128MB) physical
memory and 720MB Virtual memory.

If we look in monitor we see that there is
only 65% used from the physical memory and
92% of the virtual memory.
Before we went to 192MB we had 128MB and the
used percentage of the memory where both (for
physical and virtual memory) about 95%.

Why is  the system not using first the
physical memory and then the virtual memory?
Can we force processes to use more physical
memory?

thanks for any help

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