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Is there any meaningful way to compare two Time Travel Debugging
positions?

[png]

Raymond Chen

September 5th, 20220 0

   

Time Travel Debugging uses position objects to represents points in
the recorded debugging trace. A position object consists of two
numbers (represented in hexadecimal) separated by a colon. The first
number is a 64-bit number known as the sequence number, and the
second number is a 32-bit number known as the step number.

So if you have two position objects, is there a way to determine
which one occurred first in the trace?

Generally speaking, the position objects can be sorted
chronologically by sequence number, and then by step within a
sequence number.1 This ordering is valid even across traces from
different processes running simultaneously on the same system. (The
consistent cross-process ordering is handy when you are debugging a
problem that spans multiple processes.)

Roughly speaking, what you can say is that the first step (step zero)
of each sequence is chronological by sequence number. The remaining
steps in the sequence proceed chronologically within the sequence,
but chronological ordering between steps from different sequences is
a bit fuzzier because multiprocessing is like that.

For example, consider this timeline:

 

Thread 1 3:0 3:1     3:2     6:0 6:1

 

Thread 2 4:0 4:1 4:2 4:3 5:0 5:1

At the start of the trace, thread 1 is assigned sequence number 3,
and thread 2 is assigned sequence number 4. Thread 1 then executes
instructions 3:0, 3:1, and 3:2. Meanwhile, thread 2 executes
instructions 4:0, 4:1, and 4:2. Instructions do not all take the same
amount of time to execute, so you can't compare step numbers between
sequences. In the above example, thread 1 executed some slow
instructions, so it is stuck on step 2 (within sequence 3). while
thread 2 is already on step 3 (within sequence 4).

In the above trace, thread 2 concluded sequence 4 after four
instructions, and its next sequence was assigned the number 5. A
little while later, thread 1 concluded sequence 3 and it received
sequence number 6 for its next sequence of instructions.

So how does the Time Travel Debugger decide when to end a sequence
and start a new one?

Sequence boundaries occur at these points:

  * The thread issues a synchronizing instruction, such as an
    interlocked memory access or a memory barrier.
  * The thread transitions to kernel mode, say, because of a system
    call.
  * At the Time Travel Debugger's discretion. (Say, because the
    sequence is getting kind of long.)

Don't be surprised when you see the sequence number jump forward a
lot when you step over a system call. After all, that system call
could have taken a long time, and other threads got to run thousands
of sequences while the thread you're stepping through was stuck in
the system call.

Bonus chatter: The Time Travel Debugging folks tell me that the
realities of multiprocessing can result in apparent contradictions in
the Time Travel Debugging trace. For example, if two threads access
the same memory without synchronization, they may observe values that
appear to be contradictory with respect to each other. You might
observe one thread write a value to memory, and then another thread
reads from that memory and gets the old value! This problem can be
exacerbated by false sharing.

1 This is what we did some time ago when we studied the case of the
mysterious over-release from deep inside the marshaling
infrastructure.

[png]

Raymond Chen

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