I've instructed myself a bit on how to use gdb and went after the bug.
Here's a stack trace of the bug in action:
(gdb) bt
#0 0x00007ffff31a9c53 in __select_nocancel ()
at ../sysdeps/unix/syscall-template.S:82
#1 0x00007ffff7b6cd69 in pkgAcquire::Run (this=0x7fffffffc830,
PulseIntervall=<value optimized out>) at acquire.cc:346
#2 0x00007ffff7b666c2 in ListUpdate (Stat=<value optimized out>, List=...,
PulseInterval=5000) at algorithms.cc:1367
#3 0x00000000004653a9 in RPackageLister::updateCache (
this=<value optimized out>, status=0xa80d00, error=...)
at rpackagelister.cc:1301
#4 0x0000000000440148 in RGMainWindow::cbUpdateClicked (
self=<value optimized out>, data=0x75ba70) at rgmainwindow.cc:2999
What's happening here is that synaptic is using the apt libraries to run
a tight loop using select to update the progress bar status. In
RPackageLister::updateCache, line 1301 is:
bool res = ListUpdate(*status, *_cache->list(), 5000);
That eventually gets used as tv.tv_usec in a select() call making this a
select loop where each select only waits for 0.005 seconds. I tried
bumping that value to 500000 (0.5 seconds) and it solves the CPU problem
but because the code is single threaded it slows down the whole
interface. A more involved patch is needed.
--
Update Manager causes high Xorg CPU usage when checking for updates
https://bugs.launchpad.net/bugs/355355
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