Re^6: Program Design Around Threads

[aeaton1843]

Please don't take this the wrong way. You are hitting me with the clue stick and I want to make sure I understand everything here.

As I understand your code, it gives each machine in the machines list a thread and then runs through each of the commands in the commands list sequentially.

What I don't understand is why your statement: "Making multiple connections to each machine from different threads to run single commands is both inefficient and the source of your problems." is true. Except for the last part because we both agree there. When I look at it, let's say I have three commands per device each of which take on average 15 seconds to retrieve. If I create a thread for each command, I wait 15 seconds to finish all three commands for that machine. That is, of course, given a high enough allowed running thread count. If I do it your way I no longer have the synch issues but I am now waiting an extra 30 seconds to finish each machine. Granted you have 10 machines threads running sequentially.

We agree about the source of my problems. I am trying to further wrap my head around why saving a possible 30 seconds per device in this scenario was a less than optimal approach. That is other than the fact that it causes me a lot of synchronization issues. So be it if that is the answer. At least I found out the right way to go about the problem even if it takes a bit longer to get all the output.

I do appreciate the comments.

[BrowserUk]

I am trying to further wrap my head around why saving a possible 30 seconds per device in this scenario was a less than optimal approach. That is other than the fact that it causes me a lot of synchronization issues.

Okay. Using your numbers: 100 machines; 3 commands; 15 seconds per command; and 10 concurrent threads.

• Your way;

  You process 10 commands (3 1/3 machines) every 15 seconds: 100 / 3.333 * 15 / 60 = 7.5 minutes.

• My way:

  I process 10 machines every 45 seconds: 100 / 10 * 45 / 60 = 7.5 minutes.

But: I've spawned 100 threads and made 100 connections. No locking, nor waiting, nor syncing to slow things down.

You've spawned 300 threads and made 300 connections. And you had to acquire locks and wait for them.

Given the IO bound nature of the problem, the locking might not slow you down too much -- assuming that you can get it right without creating dead-locks; live locks or priority inversions et al. -- but you've definitely consumed 2 or 3 times as much cpu; caused 3 times as much network traffic; 3 times the load on the remote machines; and consumed more memory; to achieve the same overall elapsed time.

It just isn't worth the hassle.

[aeaton1843]

Heh :-) I should have put the math down on paper first in those terms. I didn't believe it until I saw it. Even if I spawn all 300 threads to your 100 I only save 30 seconds on your code. For all those processes and resources, you're right, not worth it. For some reason, it didn't click until now. My myopic vision has been cleared. I am grateful for your time, many thanks.