IA64 Linux VM performance woes.

[Michael E. Thomadakis]

Hello all.

We are trying to deploy a 128 PE SGI Altix 3700 running Linux, with 265GB main
memory and 10TB RAID disk (TP9500) :

# cat /etc/redhat-release
Red Hat Linux Advanced Server release 2.1AS (Derry)

# cat /etc/sgi-release
SGI ProPack 2.4 for Linux, Build 240rp04032500_10054-0403250031

# uname -a
Linux c 2.4.21-sgi240rp04032500_10054 #1 SMP Thu Mar 25 00:45:27
PST 2004 ia64 unknown

We have been experiencing bad performance and downright bad behavior when we
are trying to read or write large files (10-100GB).

File Throughput Issues
----------------------
At first the throughtput we are getting without file cache bypass is at around
440MB/sec MAX. This specific file system has LUNs whose primary FC paths go
over all four 2Gb/sec FC channels and the max throughput should have been
close to 800MB/sec.

I've also noticed that the FC adapter driver threads are running at 100% CPU
utilization, when they are pumping data to the RAID for long time. Is there
any data copy taking place at the drivers? The HBAs are from QLogic.


VM Untoward Behavior
-------------------
A more disturbing issue is that the system does NOT clean up the file cache
and eventually all memory gets occupied by FS pages. Then the system simply
hungs.

We tried enabling / removing bootCPUsets, bcfree and anything else available
to us. The crashes are just keep comming. Recently we started experiencing a
lot of 'Cannot do kernel page out at address' by the bdflush and kupdated
threads as well. This complicates any attempt to tune the FS in a way that can
maximize the throughput and finally setup sub-volumes on the RAID in a way
that different FS performance objectives can be attained.


Tunning bdlsuh/kupdated Behavior
-------------------------------

One of our main objectives at our center is to maximize file thoughput for our
systems. We are a medium size Supercomputing Center were compute and I/O
intensive numerical computation code runs in batch sub-systems. Several
programs expect and generate often very large files, in the order of 10-70GBs.
Minimizing file access time is importand in a batch environment since
processors remain allocated and idle while data is shuttled back and forth
from the file system.

Another common problem is the competition between file cache and computation
pages. We definitely do NOT want file cache pages being cached, while
computation pages are reclaimed.

As far as I know, the only place in Linux that the VM / file cache behavior
can be tuned is with the 'bdflush/kupdated' settings. We need a good way to
tuneup the 'bdflush' parameters. I have been trying very hard to find in-depth
documentation on this.

Unfortunately I have only gleaned some general and abstract advices on the
bdflush parameters, mainly in the kernel source documentation tree
(/usr/src/kernel/Documentation/).

For instance, what is a 'buffer'? Is it a fixed size block (e.g., a VM page)
or it can be of any size? This is important as bdlush uses number and
percentages of dirty buffers. A small number of large buffers require much
more data to get transferred to the disks, vs. a large number of small
buffers.

Controls that are Needed
------------------------
Ideally we need to:

1. Set an upper bound on the number of memory pages ever caching FS blocks.

2. Control the amount of data flushed out to disk in set time periods; that is
we need to be able to match the long term flushing rate with the service rate
that the I/O subsystem is capable of delivering, tolerating possible transient
spikes. We also need to be able to control the amount of read-ahead, write
behind or even hint that data are only being streamed through, never to be
reused again.

3. Specify different parameters for 2., above, per file system: we have file
systems that are meant to transfer wide stripes of sequential data, vs. file
systems that need to perform well with smaller block, random I/O, vs. ones
that need to provide access to numerous smaller files. Also, cache percentages
per file system would be useful.

4. Specify, if else fails, what parts of the FS cache should flushed in the
near future.

5. Provide in-depth technical documentation on the internal workings of the
file system cache, its interaction with the VM and the interaction of XFS/LVM
with the VM.

6. We do operate IRIX Origins and IBM Regatta SMPs where all these issues have
been addressed to a far more satisfying degree than on Linux. Is the IRIX file
system cache going to be ported to ALTIX Linux? There is already a LOT of
experience in IRIX for these types of matters that should NOT remain
unleveraged.


Any information/hint or pointers for in-depth discussion on the bugs and
tunning of VM/FS and I/O subsystems or other relevant topics would be
GREATLY appreciated!

We are willing to share our experience with anyone who is interested in
improving any of the above kernel sub-systems and provide feedback with
experimental results and insights.

Thanks

Michael Thomadakis

Supercomputing Center
Texas A&M University
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