[linuxperf] Q&A

[John Justice <justice@quantumres.com>]

Here's some info culled from the kernel mailing list.  I'm not sure what
format we should use here.  Should we credit the authors on every question
and answer, or are kernel posts open to cleanup and reposting with edits?
Also, this is just a quick batch of Q&As I crufted together.  None of the
material is checked for accuracy, etc.

Comments?

************************************
Network performance:
************************************
Question:
> We are using a PC running linux 2.0.34 in a data collection application
> to transfer data from a Windows95 PC. We have a 100Base-T network and
> our tcp/ip performance between these machines is quite poor (about 12
> mbits/sec). We tried 3 ethernet cards and 4 drivers. However when we
> upgraded to version 2.1.131 of the kernel the network performance
> improved substantially (about 40 mbits/sec). This is with the same
> network driver. I checked the Changes file for 2.1, but there was
> nothing obvious which would explain the performance improvement. Does
> anyone have any idea what could have changed in the kernel to account
> for this?

Answer:
The network stack in 2.2 is a near complete rewrite compared to 2.0.  TCP
has been completely reimplemented by Pedro Roque and David Miller to fix
some architectural problems in the 2.0 code base. The IP layer had lots of
major changes too, for example it uses a "virtual destination cache" now to
streamline the kernel code paths, the routing code has been completely
rewriten etc.

That is why it is called NET4, not NET3 anymore.

-Andi 

Question:
On my Dec Alpha squid box, ping time on the local subnet increases with
time. i.e. network throughput slowly but surely decreases; from a ping RTT
of 3ms to 3000ms from another PC on the same subnet. This 1000 fold
degradation typically takes from a couple of hours to maybe a day depends
on traffic. After which the alpha boxes are totally inaccessible either via
ftp or telnet.  

Answer:
Try upgrading to the latest tulip driver. Versions before v.90q were
susceptible to problems like this.

Question:
My ppp connection is terrible.  I seems to be dropping packets all over the
place.

Answer:
First, make sure your MTU is set to 1500.

Then //need more content here

************************************
Filesystem performance
************************************

Question:
I'm having terrible performance on my 2.0.x machine when running GNU find.
What can I do to improve this?

Answer:
1. First, try using the most recent 2.2.x kernel.  There were some huge
performance gains made in filesystem object caching for the 2.2 series

2. Then, try `chattr -R +A directory', which turns off atime updating on
the directory tree.  That makes a huge difference for large trees.

3. If you're really ambitious, consider writing a program to scan the tree
in a better inode order, as getdents() returns the inode numbers.  GNU find
does not use this information.

************************************
Sound Performance
************************************

Question:
My sound card seems to cut out under heavy load.  xosview shows timer and
scsi interrupts going on while sound interrupts make a pause during the
dropout.

Answer:
This can be very easily explained by the application using sound being
stalled and thus no longer feeding sound data.  Therefore think about the
sound interrupts stopping as a secondary effect rather than the cause.  Try
renicing the sound application to a higher priority.

Question:
My mp3 player has started skipping on my smp box under 2.2.3 when I compile
the kernel.  In 2.0.x I never had this problem.  Any advice?

Answer:
The 2.2.x scheduler the same as the 2.0.x scheduler except for the
improvements in smp.  These cause applications to sometimes jump between
cpus during times of heavy load.  This can sometimes cause mp3 players to
skip.  Try renicing the mp3 player to a higher priority.
//does this still happen with newer kernels than 2.2.3? 
***********************************************************
* John Justice              ** justice@quantumres.com     *
* Quantum Research Services ** http://www.quantumres.com  *
* Voice (919)544-4952       ** Fax: (919) 544-2918        *
***********************************************************

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