Shared SO's and Load Balancing

Question

On Wed, 21 Jan 1998 17:24:33 -0000, Richard Stobart wrote:
Dale,
If SHARED is true in a Service Object then the Service Object is not
re-entrant (because many clients are sharing its variables and therefore is
not multitasked (Funny logical naming if you ask me)). I got a lot of replies correcting me on what SHARED means! I do know
what SHARED means, I just phrased it backwards in my post. Excuse me
while I smack myself upside the head. :)
All replicates of a
load balanced partition are not re-entrant and thus equivalent to SHARED =
true. The advantage of load balancing is that the replicates can be
distributed over machines and thus the load is balanced. What I fail to understand is this: If you have a non-SHARED SO all by
itself in a partition which is not load-balanced, it will be
re-rentrant and multiple users can call it at the same time. But as
soon as you load-balance it, all of a sudden it behaves as if it were
SHARED. Why? I don't understand the technical limitations that impose
this, nor do I understand the advantage. For example, let's say that I
have 50 concurrent users of the SO I described above. It's in a
non-load-balanced partition, so all 50 users can access it at the same
time without any problem. Now let's say my server is a little
stressed, so I decide I want to load-balance my SO and have two
replicates, one on the original server and one on a second server. But
now that I've load-balanced it, the partitions act as if they're SHARED
and my 50 concurrent users are going to be lining up in queues and
suffering from horrible response times. How is this advantageous?
================================================
Dale V. Georg
Systems Analyst
Indus Consultancy Services
[email protected]
================================================

Answer 1

Dale,
The argument for load balancing services falls into two basic categories:
1. The service is blocking. That is, it is either not thread safe, not
signal tolerant, or not re-entrant. Most database libraries are the most
common example of this. Any partition that contains a non-multi-threaded
DB library will block the entire partition. Thus, you MUST load balance.
2. You want to spread the load across multiple servers. This is a tricky
one - you really have to do load testing to see if you're getting the
benefits you think you are.
Load balancing vs. multi-threading is a pretty hot topic among all of the
clients I visit. On a machine with native or certified POSIX threads and
multiple CPUs, you will certainly gain significant performance by not
load-balancing a non-blocking partition. Compare this with shared and/or
transactional services within a partition, non-native or non-POSIX thread
partitions, and machines with one CPU, and you have yourself quite a pot of
fish soup. Add another machine with differing properties, and the effects
can be exponential.
Then, there is also the notion that load balanced services are
intrinsically single threaded, which is not true. They can, and often do,
act as single threaded services because of the router. But it doesn't
imply that either the router, nor the partitions it routes, are necessarily
single threaded.
Don
At 07:38 PM 1/21/98, Dale V. Georg wrote:
On Wed, 21 Jan 1998 17:24:33 -0000, Richard Stobart wrote:
Dale,
If SHARED is true in a Service Object then the Service Object is not
re-entrant (because many clients are sharing its variables and therefore is
not multitasked (Funny logical naming if you ask me)). I got a lot of replies correcting me on what SHARED means! I do know
what SHARED means, I just phrased it backwards in my post. Excuse me
while I smack myself upside the head. :)
All replicates of a
load balanced partition are not re-entrant and thus equivalent to SHARED =
true. The advantage of load balancing is that the replicates can be
distributed over machines and thus the load is balanced. What I fail to understand is this: If you have a non-SHARED SO all by
itself in a partition which is not load-balanced, it will be
re-rentrant and multiple users can call it at the same time. But as
soon as you load-balance it, all of a sudden it behaves as if it were
SHARED. Why? I don't understand the technical limitations that impose
this, nor do I understand the advantage. For example, let's say that I
have 50 concurrent users of the SO I described above. It's in a
non-load-balanced partition, so all 50 users can access it at the same
time without any problem. Now let's say my server is a little
stressed, so I decide I want to load-balance my SO and have two
replicates, one on the original server and one on a second server. But
now that I've load-balanced it, the partitions act as if they're SHARED
and my 50 concurrent users are going to be lining up in queues and
suffering from horrible response times. How is this advantageous?
================================================
Dale V. Georg
Systems Analyst
Indus Consultancy Services
[email protected]
================================================
============================================
Don Nelson
Regional Consulting Manager - Rocky Mountain Region
Forte Software, Inc.
Denver, CO
Phone: [phone]
Corporate voice mail: 510-986-3810
aka: [email protected]
============================================
"We tigers prefer to inflict excitement on others." - Hobbes