Terje Eggestad wrote:
> On a single box you would use a shared memory segment to do this. It has
> the following advantages:
> - no syscalls at all
Unless you poll for messages on the receiving side, how do you trigger
the receiver to look for a message? Shared memory doesn't have file
> - whenever the recipients need to use the info, they access the shm
> directly (you may need to use a semaphore to enforce consistency, or if
> you're really pressed on time, spin lock a shm location) There is no
> need for the recipients to copy the info to private data structs.
How do they know the information has changed? Suppose one process
detects that the ethernet link has dropped. How does it alert other
processes which need to do something?
> Why does it help you to know that there are no recipients contra the
> wrong number recipients ???? OR asked differently, if you don't have a
> notion of who the recipients are/should be, why would you care if there
> are none??????
> There are practically no real applications for this feature.
It's true that if I have a nonzero number of listeners it doesn't tell
me anything since I don't know if the right one is included. However,
if I send a message and there were *no* listeners but I know that there
should be at least one, then I can log the anomaly, raise an alarm, or
take whatever action is appropriate.
> Also: Keep in mind that either you do multicast, or explisit send to
> all, the data you're sending are copied from you buffer to the dest
> sockets recv buffers anyway. If you're sending 1k you need somewhere
> between 250 to 1000 cycles to do the copy, depending on alignment. I've
> measured the syscall overhead for a write(len=0) to be about 800 cycles
> on a P3 or athlon, and about 2000 on P4. If you really have enough
> possible recipients, you should use a shm segment instead. If you have
> only a few (~10) the overhead is worst case 20000 cycles, or on a 2G P4,
> 10 microsecs to do a syscall for each. Who cares...
Granted, shared memory (or sysV message queues) are the fastest way to
transfer data between processes. However, you still have to implement
some way to alert the receiver that there is a message waiting for it.
For large packet sizes it may be sufficient to send a small unix socket
message to alert it that there is a message waiting, but for small
messages the cost of the copying is small compared to the cost of the
context switch, and the unix multicast cuts the number of context
switches in half.
-- Chris Friesen | MailStop: 043/33/F10 Nortel Networks | work: (613) 765-0557 3500 Carling Avenue | fax: (613) 765-2986 Nepean, ON K2H 8E9 Canada | email: email@example.com
- To unsubscribe from this list: send the line "unsubscribe linux-net" in the body of a message to firstname.lastname@example.org More majordomo info at http://vger.kernel.org/majordomo-info.html
This archive was generated by hypermail 2b29 : Fri Mar 07 2003 - 22:00:00 EST