Background tasks even with idle priority hog disk access

I frequently (several times a day) find that my computer becomes almost, and often entirely, unusable for day-to-day tasks because of bouts of disk thrashing. Usually this is not due to swapping but to some background task. This happens even when iotop says that the task in question has idle priority.

Example:
This morning I was editing a wiki in Firefox when suddenly the disk light came on and I couldn't type or even move the mouse pointer. Several minutes later I came back and finally managed, over a period of several minutes, to switch to a terminal window running iotop, where I saw there was a "find /" process doing huge disk reads. This process had idle priority.

What should happen:
Tasks with idle priority should yield to more urgent tasks, such as normal desktop applications, so that the user is not forced to abandon the computer until the background task is finished.

ProblemType: Bug
DistroRelease: Ubuntu 13.10
Package: linux-lowlatency 3.11.0.14.5
ProcVersionSignature: Ubuntu 3.11.0-14.6-lowlatency 3.11.7
Uname: Linux 3.11.0-14-lowlatency x86_64
ApportVersion: 2.12.5-0ubuntu2.1
Architecture: amd64
Date: Thu Dec 12 08:33:13 2013
EcryptfsInUse: Yes
InstallationDate: Installed on 2013-08-22 (111 days ago)
InstallationMedia: Ubuntu-Studio 13.04 "Raring Ringtail" - Release amd64 (20130424)
MarkForUpload: True
SourcePackage: linux-meta-lowlatency
UpgradeStatus: Upgraded to saucy on 2013-10-20 (52 days ago)