Messages in this thread | | | Date | Sun, 06 Feb 2005 04:25:42 +0000 | From | Nuno Silva <> | Subject | Re: Drive performance bottleneck |
| |
Andrew Morton wrote: > Lincoln Dale <ltd@cisco.com> wrote: > >>sg_dd uses a window into a kernel DMA window. as such, two of the four >>memory acccesses are cut out (1. DMA from HBA to RAM, 2. userspace >>accessing data). >>1.6Gbps / 2 = 800MB/s -- or roughly what Ian was seeing with sg_dd. > > > Right. That's a fancy way of saying "cheating" ;) > > But from the oprofile output it appears to me that there is plenty of CPU > capacity left over. Maybe I'm misreading it due to oprofile adding in the > SMP factor (25% CPU on a 4-way means we've exhausted CPU capacity).
sg_dd is lying or /dev/sg* is broken. Try to do that sg_dd test in any single drive and you'll get 20 times the performance you're supposed to achieve:
puma:/tmp/dd# time sg_dd if=/dev/sg1 of=/dev/null bs=64k count=1000000 time=1 Reducing read to 64 blocks per loop time to transfer data was 69.784784 secs, 939.12 MB/sec 1000000+0 records in 1000000+0 records out
This is a single sata drive. I'm lucky, am I not? ;-)
Regards, Nuno Silva
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |