1346927 - Replace PVRManager::GetSensorState with lock-free shared memory structure (Eliminating message queue IPC)

Status: NEW

(Core :: WebVR, enhancement)

Description

[:kip (Kearwood Gilbert)]

A lock-free structure in shared memory is more efficient than an IPC message queue, will help reduce latency, and will eliminate the sync IPC message in support of Bug 1344216.

Comment 1

[:kip (Kearwood Gilbert)]

For some background on the lock-free approach, see "Concurrent reading and writing" by Leslie Lamport:

http://dl.acm.org/citation.cfm?doid=359863.359878

This is applicable to VR headset state, such as positional tracking data, as it is updated by a single thread while being read by multiple threads.  Concurrent writing by multiple threads is not necessary.

Comment 2

[:kip (Kearwood Gilbert)]

(In reply to :kip (Kearwood Gilbert) from comment #1)
> For some background on the lock-free approach, see "Concurrent reading and
> writing" by Leslie Lamport:
> 
> http://dl.acm.org/citation.cfm?doid=359863.359878
> 
> This is applicable to VR headset state, such as positional tracking data, as
> it is updated by a single thread while being read by multiple threads. 
> Concurrent writing by multiple threads is not necessary.

Specifically, I would apply the "General Readers/Writer Solution" for this application.

Comment 3

[:kip (Kearwood Gilbert)]

We will also need to ensure that the solution is tolerant of preemption and does not result in crashing the content process if the GPU process is killed.

Comment 4

[:kip (Kearwood Gilbert)]

(In reply to :kip (Kearwood Gilbert) from comment #3)
> We will also need to ensure that the solution is tolerant of preemption and
> does not result in crashing the content process if the GPU process is killed.
Pre-emption effects can be avoided by limiting the number of iterations attempting to get a new headset state in the reader.  When a valid state is read, it will be copied to the reader's non-shared memory.  In the event of the writer being pre-empted for an extended duration of time, the reader should fall back to the prior recorded sensor states.

Comment 5

[:kip (Kearwood Gilbert)]

Attachment: GeckoProfiler timeline showing PVRManager::GetSensorState sync IPC blocking between 5ms and 66ms

Comment 6

[Daosheng Mu[:daoshengmu]]

Kip, have you taken a look at, https://dxr.mozilla.org/mozilla-central/rev/0b255199db9d6a6f189b89b7906f99155bde3726/ipc/glue/SharedMemory.h? Could it have any help for us?

Comment 7

[:kip (Kearwood Gilbert)]

(In reply to Daosheng Mu[:daoshengmu] from comment #6)
> Kip, have you taken a look at,
> https://dxr.mozilla.org/mozilla-central/rev/
> 0b255199db9d6a6f189b89b7906f99155bde3726/ipc/glue/SharedMemory.h? Could it
> have any help for us?
Yes, I think me may be able to use this class in our implementation.  It's nice to have some platform abstraction

Comment 8

[BugBot [:suhaib / :marco/ :calixte]]

The bug assignee didn't login in Bugzilla in the last 7 months.
:jimm, could you have a look please?
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