[mythtv-users] Playback freeze/lockup + kernel panic

f-myth-users at media.mit.edu f-myth-users at media.mit.edu
Wed Feb 3 08:59:26 UTC 2010

    > Date: Sun, 31 Jan 2010 09:19:51 -0700
    > From: Brian Wood <beww at beww.org>

    > On Sunday 31 January 2010 09:12:55 am steve wrote:

    > > So I guess it is a memory problem.  The thing is the memtest86 didn't turn
    > > up anything. 

    > How long did you run it? Sometimes you need to run it for a day or  more.

memtest86+ is also not a magic bullet.  You can easily have memory
problems that it cannot detect.

Two examples from my recent personal experience; lots more details
upon request (though I may have sent same in some previous message):

(a) Motherboard that flips bits (with low probability) reading -from-
    memory -only if- CPU is throttled down.  Screws machine if being
    used, say, to rsync some files (gets a few flipped bits every ten
    gig or so), but undetectable by memtest86+ because the latter runs
    at full speed and CPU is never throttled down.  Problem was data-
    pattern-dependent as well.  Certain problematic files produce the
    same md5sum every time (once file was in RAM cache) if run in a
    tight loop, but -different- sums with 10-second sleeps in between
    each run (each of which only took a few hundred ms and hence
    didn't cause the CPU to throttle up and stay that way.  Once I
    detected the problem, I could very rapidly rule out the filesystem
    (crypto FS -> flipped bits would have trashed entire swaths) and
    all I/O (same behavior via IDE, SATA, and even USB).  [Solution:
    disable throttling on that motherboard and run at full speed always.
    Biggest issue there was verifying the integrity of a couple of
    months of previous operation; what a mess but ultimately okay.]
(b) Three supposedly-identical motherboards (same production batch and
    BIOS rev, even) with differing but always not-quite-right behavior
    whenever there is more than one stick of a supposedly-compatible
    memory installed.  One won't run in dual-channel mode without
    errors.  One -must- run in dual-channel mode.  Etc.  For certain
    memory voltage/timings, memtest86+ (sometimes) detected errors.
    For many more, it never detected a thing---but copying 1-5 gig via
    NFS would cause one to hang or provoke a kernel oops (or both).
    [Solution:  lock FSB ratio 1:1 instead of the motherboard's choice
    of 6:5.  Problem instantly solved, no matter where the memory
    sticks are.  That one took months, off and on, to finally resolve.]

Cases (a) and (b) were not even the same brand of motherboard and were
purchased about 5 years apart.  All were using vendor-recommended RAM.
High-quality power supplies.  Problem did not follow sticks.  Single
stick -always- okay.  Etc.

In neither case was memtest86+ remotely helpful, even on multi-day
runs, but memory errors were very clearly happening.  Trying various
versions of memtest86+ did not improve matters.

So if you -do- get an error from memtest86+, you have a problem.
If you -don't- get an error, you actually don't know much of anything.


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