[lug] q: multithreading on smp weirdness

[George Sexton]

This is a guess, but I would say thread 2 is starved waiting for chunks.

When you say thread 1 fetches a chunk, is that I/O? I/O in bigger blocks,
(up to a point) usually does better than small chunks.


George Sexton
MH Software, Inc.
http://www.mhsoftware.com/
Voice: 303 438 9585
  

> -----Original Message-----
> From: lug-bounces at lug.boulder.co.us 
> [mailto:lug-bounces at lug.boulder.co.us] On Behalf Of 
> karheng at softhome.net
> Sent: Monday, November 29, 2004 7:45 PM
> To: lug at lug.boulder.co.us
> Subject: [lug] q: multithreading on smp weirdness
> 
> 
> greetings. 
> 
> i've got a test app with 2 threads processing
> several chunks of data. thread 1 fetches chunk 1
> & processes it. once done, it's relayed to thread 2.
> then thread 1 fetches & processes chunk 2, while
> thread 2 receives & processes chunk 1. this goes
> on until all chunks are processed by both threads. 
> 
> my problem is that if the chunk size is made small
> enough, the elapse time suddenly almost doubles...
> and i need these chunks to be small eventually. 
> 
> more specifically:
> (chunk size * iteration count always == 6000)
> single threaded:
> chunk size of 6000 unit, 1 iteration, 12+ secs elapse time
> multi-threaded:
> chunk size of 12 unit, 500 iterations, 6+ secs elapse time
> chunk size of 10 unit, 600 iterations, 6+ secs elapse time
> chunk size of 8 unit, 750 iterations, 6++ secs elapse time
> chunk size of 6 unit, 1000 iterations, 6++ secs elapse time 
> most of the time
> chunk size of 5 unit, 1200 iterations, 6+++ secs elapse time 
> most of the 
> time, 11+ secs at other times.
> chunk size of 4 unit, 1500 iterations, 11+ secs elapse time
> chunk size of 3 unit, 2000 iterations, 11++ secs elapse time
> chunk size of 2 unit, 3000 iterations, 11+++ secs elapse time
> chunk size of 1 unit, 6000 iterations, 12+ secs elapse time
> (note that in above, elapse times of 7, 8, 9 & 10 secs are
> mostly not present, just 1 or 2 sporadic ones). 
> 
> i ran this on a 4 CPU itanium linux server. 
> 
> this problem doesn't appear when i ran a port
> of this app on a windows hyperthreading machine. 
> 
> anyone might have any idea what's wrong?
> thanks in advance. 
> 
> rgds, 
> 
> kh 
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