On 04/14/2010 11:23 PM, Brian Lavender wrote: >> What gets ugly is if you have 2 or more clients accessing 2 or more >> files. Suddenly it becomes very very important to intelligently handle >> your I/O. Say you have 4 clients, reading 4 ISO files, and a relatively >> stupid/straight forward I/O system. Say you read 4KB from each file (in >> user space), then do a send. > > That's like operating a disk in PIO mode. I would think something that > can leverage the DMA controller better would avoid thrashing.
DMA controllers make it easier on the kernel, but it doesn't change the nature of random I/O limiting performance. > Perhaps > there is a way to tell apache to use memory cache and dump readaheads > into memory. Linux should be able to handle this fine. I didn't see anything. > I was looking to see if there is a config on this, but I can't find one > at the moment. I was looking at this cache page. > > http://httpd.apache.org/docs/2.2/mod/mod_mem_cache.html Yeah, I looked around, it seemed to be rather focussed on having objects smaller than cache. If you want to quantify the problem I'd just download a 4GB file and measure the bandwidth. Then access enough files so that they sum to twice as big as ram. Say 4 4GB files if you have 8GB ram, or 8 4GB files if you have 16. Launch a client each, then when they get half way done fire another download for each file. If you are saturating the network, or at least within 75% of so it sounds like linux is prefetching and/or using a large enough transfer size so that random I/O isn't killing performance. If you are at 5% or less then I'd continue looking for some tuning. > I imagine someone else has encountered this problem before. I seem to recall some GigE connected freebsd box that was one of the busiest ftp servers in the world working on this kind of thing. I saw a few published articles on the software, hardware, and kernel tuning. I look around for apache tuning for large file transfers and didn't really find much. Maybe it's not a problem. > > brian _______________________________________________ vox-tech mailing list vox-tech@lists.lugod.org http://lists.lugod.org/mailman/listinfo/vox-tech
