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[warmcat <709245@xxxxxxxxxxxxxxxxxx>]

Maybe the 1ms threshold thing is just to do with a threshold heuristic
where it stops using the scheduler and sleeping and instead spins
udelay() style, hence the 100% CPU.  That's not to write off its effect
on the bug just a possible explanation of why it cares about 1ms or
less.

Reading these new findings yesterday and today I wonder if it's somehow
related to when the ehci stuff schedules the URB.  Like if the CPU is
totally up it can pack USB transactions to go out immediately, but
somehow when "idle", although what exactly is in what kind of idle is
unclear, it bumps it to the next frame or so.

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https://bugs.launchpad.net/bugs/709245

Title:
  panda: USB disk IO slow

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 Oneiric:
  Confirmed

Bug description:
  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

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