I have a machine with a good proportion of IOWait 20-30%. It does have
local disks and it performs operations on NFS mounts. I just wanted to
be sure if IOWait includes NFS activity or not. I also want a way if it
is NFS to be able to say for sure if it is a bottleneck on the nfs
client or server. NFS is not a linux machine so visibility is not
allways the best.
Grant
Daniel Pittman wrote:
Grant Street <[EMAIL PROTECTED]> writes:
I was wondering if someone could point me in the right direction for
some doco?
I don't know of any, but ...
I'm finding it hard to get a definition of what constitutes IOWait.
I know that IOwait is CPU time waiting for IO to happen to physical local
disks, but I'm unsure about the following scenarios and if they contribute to
IOWait:
Not quite. IOWait is a *software* state, indicating that a process or
thread is blocked waiting for I/O to complete.
This is different from "CPU time waiting for ..." in that it implies the
software is making no progress, but *NOT* that your CPU is spending
cycles working on it.[1]
- CPU time waiting for an NFS read/write to occur
Yes, along with more or less any other "disk" I/O that happens to be run
over the network -- as long as it is synchronous, and something is
waiting on it.
- CPU time waiting for a network buffer to be read/written to. eg waiting for
a full buffer to clear.
Generally not. I am not certain about blocking on a full buffer
condition for sending data, but not for blocking while reading.
- Anything else??
Any other synchronous disk I/O, certainly. Probably certain other,
related, conditions where the kernel developers feel that the process is
blocked on I/O.
PS. How do you set/query the network buffers in Linux?
Via the socket fcntl / ioctl interface, or via the sysctls in
/proc/sys/net, which are documented in the standard Linux kernel sysctl
documentation.
All that said, you might want to tell us why you are asking, not just
what, since I suspect there is a question about why you have so much
IOWait time on your system, or poor performance?
Regards,
Daniel
Footnotes:
[1] It does, technically, spend a few in terms of submitting and
completing the I/O before it wakes up the blocked process, and
various I/O devices need babysitting, but the principal is sound. ;)
Animal Logic
http://www.animallogic.com
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