[Ocfs2-users] High on buffers and deep on swap
Alexei_Roudnev
Alexei_Roudnev at exigengroup.com
Tue Apr 10 12:58:56 PDT 2007
Mount FS with datavolume option, if your version supports it.
Set up swapiness option in the kernel.
And use this option in the Oracle:
FILESYSTEMIO_OPTIONS
Property Description
Parameter type String
Syntax FILESYSTEMIO_OPTIONS = { none | setall | directIO | asynch }
Default value There is no default value.
Modifiable ALTER SESSION, ALTER SYSTEM
Basic No
----- Original Message -----
From: Eckenfels. Bernd
To: ocfs2-users at oss.oracle.com
Sent: Tuesday, April 10, 2007 11:11 AM
Subject: RE: [Ocfs2-users] High on buffers and deep on swap
Hello Luis,
the large usage of cache by inodes is not an issue in the typical database setup, because you have only a very limited number of files (used) on your filesystems. Also the database usually grows initially and then does not request much more additional memory in operations.
However you are right, Oracle with Data Files on ocfsv2 has no need to cache the data blocks on the FS layer. But I gues sthis is not done (memadvice, fcntl or similiar methods can be used to enforce "directio" - i am not sure which one is used by Oracle). I think the original problem reports with large usage have been due to non-db workloads.
Gruss
Bernd
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From: ocfs2-users-bounces at oss.oracle.com [mailto:ocfs2-users-bounces at oss.oracle.com] On Behalf Of Luis Freitas
Sent: Tuesday, April 10, 2007 7:59 PM
To: Alexei_Roudnev; ocfs2-users at oss.oracle.com
Subject: Re: [Ocfs2-users] High on buffers and deep on swap
Alexei,
I am not a Linux kernel specialist, but generally memory on the "buffer cache" is on a different "list" than memory "free", so the kernel has to reclaim the buffers from the buffer "list" to release them to the "free" "list" as memory pressure increases.
This has a processing cost. And on large database servers is undesirable as the database does not use the buffer cache, so we have the processing cost without any benefit.
Usually on a database environment I tune the kernel as to keep a large amount of the memory on the "free" list, so that it be readly available when spawning new database processes.
Regards,
Luis
Alexei_Roudnev <Alexei_Roudnev at exigengroup.com> wrote:
Suni, you do know it much better - is not is strange a little? (so many ext3 inode chache objects, togetrher with 3 GB of cached disk space).
But I dont see anything wrong below - big 'cached' memory means only 'you have so many unused memory that system cached files in it' and nothing more. Cached memory is in reality _free for immediate use_ memory (can be used by anyone immediately).
What ysstem is doing is _instead of freeing memory and filling it by zerous, free unused memory and fill it in by buffer cache data, so that if someone need these data he have them at once. If someone need more memory, system reuse any of 'chached' memory without any delay (because these are buffers which are already written to the disk or which was never udpated).
Big 'cached' value means only _big free memory_.
----- O riginal Message -----
From: Luis Freitas
To: Alexei_Roudnev ; Brian Sieler ; ocfs2-users at oss.oracle.com
Sent: Monday, April 09, 2007 4:32 PM
Subject: Re: [Ocfs2-users] High on buffers and deep on swap
Alexei,
How can I relate the information on slabtop to the actual memory used by buffers?
I see this on slabtop:
Active / Total Objects (% used) : 603822 / 649643 (92.9%)
Active / Total Slabs (% used) : 47216 / 47216 (100.0%)
Active / Total Caches (% used) : 97 / 133 (72.9%)
Active / Total Size (% used) : 176601.08K / 181508.49K (97.3%)
Minimum / Average / Maximum Object : 0.01K / 0.28K / 128.00K
OBJS ACTIVE USE OBJ SIZE SLABS OBJ/SLAB CACHE SIZE NAME
202461 202451 99% 0.54K 28923 7 115692K ext3_inode_cache
232206 232153 99% 0.15K 8931 26 35724K dentry_cache
29974 26092 87% 0.27K 2141 14 8564K radix_tree_node
68250 62400 91% 0.05K 910 75 3640K buffer_head
855 855 100% 4.00K 855 1 3420K pmd
647 647 100% 4.00K 647 1 2588K size-4096
8595 7694 89% 0.25K 573 15 2292K filp
20835 17331 83% 0.09K 463 45 1852K vm_area_struct
780 767 98% 2.00K 390 2 1560K size-2048
18849 7962 42% 0.06K 309 61 1236K size-64
2440 632 25% 0.50K 305 8 1220K size-512
2926 2889 98% 0.34K 266 11 1064K inode_cache
256 256 100% 3.00K 128 2 1024K biovec-(256)
600 592 98% 1.38K 120 5 960K task_struct
515 512 99% 1.38K 103 5 824K pirpIo
5084 2365 46% 0.12K 164 31 656K size-128
The largest area is about 100Mb, but on free there are over 3Gb on the "cached" column:
[oracle at br001sv0431 ~]$ free
total used free shared buffers cached
Mem: 5190736 4461420 729316 0 141836 3265464
-/+ buffers/cache: 1054120 4136616
Swap: 2048248 0 2048248
[oracle at br001sv0431 ~]$
Regards,
Luis
Alexei_Roudnev <Alexei_Roudnev at exigengroup.com> wrote:
Did you run slabtop ? It can show unreleased buffers in the system.
----- Original Message -----
From: Luis Freitas
To: Alexei_Roudnev ; Brian Sieler ; ocfs2-users at oss.oracle.com
Sent: Monday, April 09, 2007 2:29 PM
Subject: Re: [Ocfs2-users] High on buffers and deep on swap
Alexei,
Yes, it seems to have no effect, which too is very strange. On 2.4 vm.freepages had a very easy to notice effect.
There are other people having problems with buffers not being released on the list and some of them are forcing the kernel cache to be flushed with:
echo 3 > /proc/sys/vm/drop_caches
But I dont see this parameter on RHAS 4.0.
Also, to be fair this seems to be a generic VM issue, I see this on servers that are not running ocfs2 too. And I only see this behavior on machines with more than 2Gb of memory.
Regards,
Luis
Regards,
Luis
Alexei_Roudnev <Alexei_Roudnev at exigengroup.com> wrote:
Did you tried vm.swappiness parameter?
(/proc/sys/vm/swappiness)
-----
Original Message -----
From: Brian Sieler
To: 'Luis Freitas' ; ocfs2-users at oss.oracle.com
Sent: Sunday, April 08, 2007 10:52 PM
Subject: RE: [Ocfs2-users] High on buffers and deep on swap
Luis, yes I am experiencing what appears to be a similar problem you are describing. See my post from just a few minutes ago on another thread.
I run a 2-node cluster with OCFS2/RAC on 2.6.9-34.0.2.ELsmp (RHEL 4.0) as well.
total used free shared buffers cached
Mem: 4044496 4005516 38980 0 34108 2236636
-/+ buffers/cache: 1734772 2309724
Swap: 2097144 648244 1448900
If you've uncovered anything since posting this message, please pass it along?
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From: ocfs2-users-bounces at oss.oracle.com [mailto:ocfs2-users-bounces at oss.oracle.com] On Behalf Of Luis Freitas
Sent: Friday, February 23, 2007 5:32 PM
To: ocfs2-users at oss.oracle.com
Subject: [Ocfs2-users] High on buffers and deep on swap
Hi,
This is a bit off topic, hope there is not a problem.
Anyone out there experiencing high swapping with the kernel retaining a large amount of buffers? This used to be a problem on 2.4, and I usually changed /proc/sys/vm/freepages to fix it. But on 2.6 this parameter no longer exists.
One of the servers here is holding over 3.5Gb of cache even when using over 700Mb of swap, and free memory is always low.
[oracle at br001sv0432 ~]$ free
total used free shared buffers cached
Mem: 5190736 4810880 379856 0 143032 3583868
-/+ buffers/cache: 1083980 4106756
Swap: 2048248 723064 1325184
[oracle at br001sv0432 ~]$
I am tuning /proc/sys/vm/swappiness, but this seems to have no effect at all. Changed from 60 to 10 and seems to have no effect. The server runs Oracle RAC with OCFS2.
Regards,
Luis
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