On Tue, Nov 10, 2009 at 4:12 AM, Corrado Zoccolo <czoccolo@xxxxxxxxx> wrote: > Hi, > On Mon, Nov 9, 2009 at 5:32 PM, Luis R. Rodriguez <mcgrof@xxxxxxxxx> wrote: >> >> And we tested this (reducing the min cpu freq to one less than the >> highest supported P state to avoid an FSB speed change) and it seems >> doing the steps described here did not fix the issue. But at least now >> if anyone else wants to verify this they can with some sort of >> documentaiton. >> >> So to confirm though -- we are seeing a huge performance depredation >> mainly on RX on an Intel Pine Trail platform with SpeedStep enabled on >> the BIOS. >> >> Let me get into the specifics in case anyone is able to help. The >> issue is with ath9k on RX and the CPU on C3 state requesting DMA over >> PCI-E. We typically would get about 110 Mbps with an AR9285 (single >> stream) but when SpeedStep is enabled it goes down to 25 Mbps. At the >> PCI-E level we are seeing huge latencies introduced when SpeedStep is >> used for DMA requests to the Intel root complex on the Intel Pine >> trail platform. Latencies are about 20-60 us. > > You mentioned speedstep and cpufreq, but the problem is with C3 state > and cpuidle (probably the BIOS mixes the two concepts, but we should > keep them separated). > C3 is not related to the core or FSB frequency, it is an idle state. > When in C3, the CPU is not ready to perform any operation, not just > slower, and depending on the CPU hw, it may take several us to wake up > (even 85us, on an Atom). > >> >> Is there a timeout threshold change that will cause the Intel chipset >> wait for some time after completion before going into a C3 state? Are >> there any other explanations for seeing such huge latencies on C3 >> state? > There is a patch from Arjan for the cpuidle menu governor, that may fix it. > http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-09/msg05276.html > It is already present in 2.6.32. Thanks all for your feedback -- this was determined to be a BIOS bug :) Anyway, hope you do consider the patch for inclusion. Luis -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
