Re: Multicast delays and high iowait

[H. Willstrand]

On Tue, Apr 1, 2008 at 11:17 PM, Matt Garman <matthew.garman@xxxxxxxxx> wrote:
> On Tue, Apr 01, 2008 at 10:24:19PM +0200, H. Willstrand wrote:
>  > On Tue, Apr 1, 2008 at 6:05 PM, Matt Garman <matthew.garman@xxxxxxxxx> wrote:
>  > > We're using multicast basically for some inter-processs
>  > > communication.
>  >
>  > Which protocol(-s) are in use? (UDP, IGMP, ...)
>
>  UDP.  FWIW, we're using the ACE_SOCK_Dgram and ACE_SOCK_Dgram_Mcast
>  objects from the ACE libraries (which, as far as I can tell, at
>  least in this situation, are just wrappers around system calls).
>
>     http://www.cs.wustl.edu/~schmidt/ACE.html
>
>
>  > >  We timestamp (and log, in a separate thread) all of our sends
>  > >  and receives, and do analysis on the logs.
>  >
>  > Are timestamps sent in the broadcast? If so, can the timestamps be
>  > out of sync generating the "delays"?
>
>  No, the timestamps aren't sent in the broadcast.  I just send a
>  sequence number.  So in my logging thread, I basically log something
>  like this: "sequence number n sent/received at time t".
>
>  I wrote a simple Perl script to compare the send and recv timestamps
>  logged for each sequence number.
>
>  Note that when I perform this test across two different machines,
>  the machines are sync'ed via NTP; we actually monitor clock
>  synchronization, and I can say with confidence that the clocks are
>  never more than a couple milliseconds off.  But also, I'm performing
>  these tests on the same machine (i.e. sender == receiver), so clock
>  sync issues don't apply.
>
>  Thanks again!
>  Matt
>
>

I see no reason why UDP packages can be delayed in a LAN with low or
medium utilization for several seconds, they will simply be lost.

Nor do I know if Java or other programming languages are in use, but
if so, for an example, a Java garbage collector can easily generate
this type of delays.

Br H. Willstrand
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