Terje Eggestad wrote:
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 descriptors.On a single box you would use a shared memory segment to do this. It has the following advantages: - no syscalls at all
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?- 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.
Why does it help you to know that there are no recipients contra theIt'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.
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.
Also: Keep in mind that either you do multicast, or explisit send toGranted, 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.
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...
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
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Chris Friesen | MailStop: 043/33/F10
Nortel Networks | work: (613) 765-0557
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