Messages in this thread | | | Date | Thu, 18 Apr 2002 08:44:45 +1000 | From | Lincoln Dale <> | Subject | Re: IDE/raid performance |
| |
At 02:58 PM 17/04/2002 +0200, Baldur Norddahl wrote: >It is clear that the 33 MHz PCI bus maxes out at 75 MB/s. Is there a reason >it doesn't reach 132 MB/s?
welcome to the world of PC hardware, real-world performance and theoretical numbers.
in theory, a 32/33 PCI bus can get 132mbyte/sec.
in reality, the more cards you have on a bus, the more arbitration you have, the less overall efficiency.
in theory, with neither the initiator or target inserting wait-states, and with continual bursting, you can achieve maximum throughput. in reality, continual bursting doesn't happen very often and/or many hardware devices are not designed to either perform i/o without some wait-states in some conditions or provide continual bursting.
in short: you're working on theoretical numbers. reality is typically far far different!
something you may want to try: if your motherboard supports it, change the "PCI Burst" settings and see what effect this has. you can probably extract another 20-25% performance by changing the PCI Burst from 32 to 64.
>Second, why are the md devices so slow? I would have expected it to reach >130+ MB/s on both md0 and md1. It even has spare CPU time to do it with.
you don't mention actually what your motherboard or chipset actually is -- and where the 32/33 and 64/66 PCI connect in. you also don't mention what your FSB & memory clock-speed are, or how these are connected to the PCI busses.
it is likely that you have a motherboard where the throughput between PCI to memory will also contend with the FSB. given you're using "time dd if=/dev/hdo1 of=/dev/null bs=1M count=1" as your test, you're effectively issuing read() and write() system-calls from user-space to kernel. this implies a memory-copy. count the number of times you're doing a memory-copy (or, more correctly, moving data across the front-side-bus), and you should be able to see another reason for the bottlenecks you see.
cheers,
lincoln.
- 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/
| |