> Erm... far be it from me to argue with the master on the > performance of the actual physical hardware, but isn't the > point here that they've generated a worst-possible-case > scenario (is this called a degenerate case these days?)
>I think that the word you're searching for is >"pathological". Its only pathological in the sense that is forces the condition to emphasize its effect. We did not start with this case. What we did notice is that a data base random write was much slower than a data base random read for a large data base. Examination showed that no I/O was actually taking place because of the Linux caching of buffers and retention of dirty buffers. (and as I said before, no paging). We first eliminated the I/O entirely by reading/writing to /dev/shm and saw the same effect - the random reads were much slower than the sequential reads to the data base. To further identify the problem, we eliminated the I/O code itself and changed it to do stores and reads from memory. We got similar results. After rearranging the code further, we ended up with the so-called "pathological" case, which tracks the elapsed time curves we were seeing originially, but showed the cause more dramatically. I think you have to re-examine what you might think to be "worst case" scenario when you start doing things like running many EC machines where the cummulative effect is to overwhelm L1/L2 cache or if you are trying to use code that has bad locality of reference for large data bases. This may also have implications in writing kernel code for the zSeries when trying to get the best possible performance. Throwing more hardware at it won't solve this poor performance problem. Adding CP's or memory may aggravate it! The only solution that I see is reducing locality of reference (working set size). And if you are trying to improve the performance of your systems, you may be overlooking what may be a major contributor - the assumptions of the hardware architect vs the assumptions of the software designer. ===== Jim Sibley Implementor of Linux on zSeries in the beautiful Silicon Valley "Computer are useless.They can only give answers." Pablo Picasso __________________________________ Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software http://sitebuilder.yahoo.com
