I tried to gather more data on this and made several runs of "hdparm -Tt" for the good and bad case: - For the good case (nosmp) - cached reads avg. 184.61MB/sec (MIN=181.02 MAX=186.30 STDDEV=1.62) - buffered reads avg. 3.64MB/sec (MIN=3.64 MAX=3.65 STDDEV=0) - For the bad case - cached reads avg. 62.00MB/sec (MIN=21.98 MAX=111.55 STDDEV=27.97) - buffered reads avg. 1.35MB/sec (MIN=1.09 MAX=1.86 STDDEV=0.20)
So not only the SMP case is slower, there is also a high variation in the numbers. Whatever happens, it is not a linear slowdown. When trying various options maxcpus=1 has the same effect as nosmp (disabling a core later does not improve things), there was one option (which right now slips my memory) causing a higher rate of timer interrupts which lead to a even worse buffered reads performance (as was disabling the irqbalance daemon). Using nohlt seemed to have no effect. When looking at the interrupts, the main difference seemed to be the use of gp_timer when booted with nosmp or maxcpus=1 and LOCal timers in the other case. Booting with 2 CPUs and disabling one did not change that. I cannot recall whether the IPIs stopped being incremented completely or only on the other CPU. But LOC definitely was still used for CPU#0. -- You received this bug notification because you are a member of Linaro Release Team, which is subscribed to the bug report. https://bugs.launchpad.net/bugs/709245 Title: ARM SMP scheduler performance bug Status in Linaro Ubuntu Evaluation Builds: Confirmed Status in Linaro Linux: Confirmed Status in “linux-ti-omap4” package in Ubuntu: Confirmed Status in “linux-ti-omap4” source package in Maverick: Confirmed Status in “linux-ti-omap4” source package in Natty: Confirmed Status in “linux-ti-omap4” source package in Oneiric: Confirmed Bug description: Original Bug name: "panda: USB disk IO slow" My Panda's USB seems to be significantly slower than a Beagle C4. hdparm shows buffered reads as ~12MB/s on the Panda, and about ~20-25MB/s on a Beagle C4 from the same external Lacie USB disk. Kernel is 2.6.37-1002-linaro-omap Disk shows as: [ 5.170440] scsi 0:0:0:0: Direct-Access LaCie d2 quadra PQ: 0 ANSI: 4 [ 5.172546] sd 0:0:0:0: Attached scsi generic sg0 type 0 [ 5.175415] sd 0:0:0:0: [sda] 976773168 512-byte logical blocks: (500 GB/465 GiB) The board is otherwise idle during the test. Doing perf_2.6.37-12 record -a dd if=/dev/sda of=/dev/null bs=4096 count=100000 shows : 81.41% swapper [kernel.kallsyms] [k] default_idle 6.33% dd [kernel.kallsyms] [k] __copy_to_user 0.94% swapper [kernel.kallsyms] [k] cpu_idle 0.51% dd [kernel.kallsyms] [k] __make_request 0.51% perf_2.6.37-12 [kernel.kallsyms] [k] __copy_from_user which suggests it's not CPU constrained. Dave To manage notifications about this bug go to: https://bugs.launchpad.net/linaro-ubuntu/+bug/709245/+subscriptions _______________________________________________ Mailing list: https://launchpad.net/~linaro-release Post to : [email protected] Unsubscribe : https://launchpad.net/~linaro-release More help : https://help.launchpad.net/ListHelp
