Hi,
The current load balancing algorithm used by the XREQ socket is
defined in lb.cpp. Requests are simply round robin'ed through the
active pipes:
current = (current + 1) % active
The problem with this approach is that it has a potential severe
bottleneck. Let's say every N request takes a very long time to
process, but all others are quick to process. Let's say there are A
active clients to the XREQ sockets that will do the processing.
If A==N (or they are integer multiples), then the same client will
always get the request that takes a long time to request. Here is the
simple example with A=N=2:
requests = [short, long, short, long, short, long, ...]
client 1 will process = [short, short, short, ...]
client 2 will process = [long, long, long, long,...]
I am wondering if we can do better than this. My idea is to pick a
client at random, rather that walking through them sequentially. Here
is a patch, that I think might implement this:
diff --git a/src/lb.cpp b/src/lb.cpp
index ca93ba2..f88fa18 100644
--- a/src/lb.cpp
+++ b/src/lb.cpp
@@ -22,6 +22,7 @@
#include "lb.hpp"
#include "pipe.hpp"
#include "err.hpp"
+#include <stdlib.h>
zmq::lb_t::lb_t () :
active (0),
@@ -90,7 +91,7 @@ int zmq::lb_t::send (zmq_msg_t *msg_, int flags_)
// continue round-robinning (load balance).
if (!more) {
pipes [current]->flush ();
- current = (current + 1) % active;
+ current = random() % active;
}
// Detach the message from the data buffer.
I don't fully understand how the active sockets and flushing works, so
this might not be a full implementation. But, I wanted to get
feedback on this idea. There are some issues that might appear
though:
* You probably want to make sure that current changes with each iteration.
* For small numbers of active clients, and small number of requests,
statistical fluctuations could lead to unequal load balancing.
Thoughts?
Cheers,
Brian
--
Brian E. Granger, Ph.D.
Assistant Professor of Physics
Cal Poly State University, San Luis Obispo
[email protected]
[email protected]
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