On Tuesday 07 August 2007 03:37:08 Alan Cox wrote: > > > acpi_pm_read is capable of disappearing into SMM traps which will make > > > it look very slow. > > > > what is an SMM trap? I googled a bit but didn't get it... > > One of the less documented bits of the PC architecture. It is possible to > arrange that the CPU jumps into a special mode when triggered by some > specific external event. Originally this was used for stuff like APM and > power management but some laptops use it for stuff like faking the > keyboard interface and the Geode uses it for tons of stuff. > > As SMM mode is basically invisible to the OS what oprofile and friends > see isn't what really occurs. So you see > > pci write -> some address > > you don't then see > > SMM > CPU saves processor state > Lots of code runs (eg i2c polling the battery) > code executes RSM > > Back to the OS > > and the next visible profile point. This can make an I/O operation look > really slow even if it isn't the I/O which is slow.
I always thought x86 is becoming a really dirty architecture. I now think it is even uglier. :-p Thank you for the thorough explanation. > > > the reason I'm talking about a "software driver limit" is because I am > > sure about some facts: > > - The disks can reach very high speeds (60 MB/s on other systems with > > udma5) > > Is UDMA5 being selected firstly ? What the kernel selects by default is udma4 (66MB/s). I tried forcing udma5 (100MB/s) with hdparm even though I think my chipset doesn't support it, and indeed there was a difference! After repetitive tests udma4 gives 20MB/s, udma5 gives 22MB/s. I'm mostly surprised however that I could even set this option. > > > So what is left? Probably only the corresponding kernel module. > > Unlikely to be the disk driver as that really hasn't changed tuning for a > very long time. I/O scheduler interactions are however very possible. I'm now trying to use the new libata driver and see what happens... Thanks, Dimitris - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
