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Re: posix message queues and multiple receivers

On 2013-12-07 20:51, David Laight wrote:
On Sat, Dec 07, 2013 at 12:38:42AM +0100, Johnny Billquist wrote:

You know, you might also hit a different problem, which I have had on
many occasions.
NFS using 8k transfers saturating the ethernet on the server, making the
server drop IP fragemnts. That in turn force a resend of the whole 8k
after a nfs timeout. That will totally kill your nfs performance.
(Obviously, even larger nfs buffers make the problem even worse.)

That wasn't the problem in this case since I could see the very delayed
That is a big problem, I've NFI why i386 defaults to very large transfers.

Even with an elevator scan algorithm and four concurrent nfs clients,
you're disk operation will complete within a few hundred ms at most.

This was all from one client. I'm not sure how many concurrent NFS
requests were actually outstanding - it was quite a few.
I remember that the operation was copying a large file to the nfs server,
the process might have been doing a very large write of an mmaped file.
So the client could easily have a few MB of data to transfer - and be
trying to do them all at once.

Thinking further, multiple nfsd probably help when there are a lot more
reads than writes - reads can be serviced from the server's cache.

Not much point in dragging this on. Without an actual system to look at, all we (I) can do is speculate. I've never seen concurrent nfs server or client been a problem. It's no different from actually having several requests outstanding to the controller at the same time on a local machine, conceptually.

If someone sees problems they think are related to this, I'd be interested in investigating it more.

For now, I'll just maintain that I do not believe that it can be a problem, but I do believe that it can be beneficial.
It's ok for me if we just disagree on this.

(And yes, I do believe that sometimes you can have problems, but the cause, and solution would not be to go single thread.)


Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email:             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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