[Webkit-unassigned] [Bug 231352] New: IPC should not have client side message hysteresis

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=231352

            Bug ID: 231352
           Summary: IPC should not have client side message hysteresis
           Product: WebKit
           Version: WebKit Local Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebKit2
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: kkinnunen at apple.com
                CC: kkinnunen at apple.com, simon.fraser at apple.com,
                    wenson_hsieh at apple.com

IPC should not have client side message hysteresis

Bug 231304 added the hysteresis to stream implementation.

The messages may have action at distance that is detectable via out-of-band observations.
At best, the hysteresis needs to be disabled for interfaces with such message behaviours.
At worst, this is not understood and results in hard to understand bugs.

Example:
- Current webgl would benefit from hysteresis
- Current webgl has prepareForDisplay() message
- If prepareForDisplay() is buffered due to hysteresis, no frame gets displayed.

Leaving a message in the client side buffer is poor form.

The problem the hysteresis is solving is;
- client side produces messages fast
- server side processes messages faster
- server side goes to sleep fast
- client side ends up spending significant time in semaphore notification


This should be solved for example with server side hysteresis wait:
When server runs out of messages, it waits with a deadline on the stream semaphore but does not set the "server is waiting flag".
To match the current hysteresis implementation, the wait time could be tuned to be roughly (current hysteresis item count * average time to produce 1 item).
When server does not have new messages messages after the hysteresis wait, it sets the "server is waiting flag" and waits on the semaphore.

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