[Webkit-unassigned] [Bug 191806] New: Web Content process main thread blocked beneath ImageDecoderAVFObjC::readSamples for many seconds on imgur.com

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=191806

            Bug ID: 191806
           Summary: Web Content process main thread blocked beneath
                    ImageDecoderAVFObjC::readSamples for many seconds on
                    imgur.com
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Media Elements
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: mitz at webkit.org
                CC: jer.noble at apple.com

Often when flipping through posts on imgur.com, the Web Content process’s main thread gets blocked for seconds beneath ImageDecoderAVFObjC::readSamples. Here’s an example (showing the trailing 3.12s of a longer hang):

  Thread 0x42edc8           DispatchQueue <multiple>  1000 samples (1-1000)     priority 0-46 (base 46)   cpu time 3.194s (12.5G cycles, 10.0G instructions, 1.25c/i)
  999  start + 1 (libdyld.dylib + 4117) [0x7fff6cbb6015]
    999  WebKit::XPCServiceMain(int, char const**) + 490 (WebKit + 1459226) [0x110dd341a]
      999  xpc_main + 433 (libxpc.dylib + 63946) [0x7fff6cf0f9ca]
        999  _xpc_objc_main + 580 (libxpc.dylib + 68983) [0x7fff6cf10d77]
          999  NSApplicationMain + 804 (AppKit + 23154) [0x7fff421e2a72]
            999  -[NSApplication run] + 764 (AppKit + 223365) [0x7fff42213885]
              999  -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 3044 (AppKit + 8224308) [0x7fff429b4e34]
                999  _DPSNextEvent + 2085 (AppKit + 268915) [0x7fff4221ea73]
                  999  _BlockUntilNextEventMatchingListInModeWithFilter + 64 (HIToolbox + 194692) [0x7fff43f6e884]
                    997  ReceiveNextEventCommon + 613 (HIToolbox + 195334) [0x7fff43f6eb06]
                      997  RunCurrentEventLoopInMode + 286 (HIToolbox + 195990) [0x7fff43f6ed96]
                        936  CFRunLoopRunSpecific + 483 (CoreFoundation + 544819) [0x7fff44c84033]
                          412  __CFRunLoopRun + 1293 (CoreFoundation + 546765) [0x7fff44c847cd]
                            412  __CFRunLoopDoSources0 + 208 (CoreFoundation + 549712) [0x7fff44c85350]
                              412  __CFRunLoopDoSource0 + 108 (CoreFoundation + 1430284) [0x7fff44d5c30c]
                                412  __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17 (CoreFoundation + 669937) [0x7fff44ca28f1]
                                  312  __NSThreadPerformPerform + 334 (Foundation + 426165) [0x7fff46dc90b5]
                                    312  WTF::dispatchFunctionsFromMainThread() + 176 (JavaScriptCore + 271520) [0x1117524a0]
                                      312  WebCore::ImageDecoderAVFObjC::readSamples() + 224 (WebCore + 2388736) [0x780247300]
                                        312  -[AVAssetReaderTrackOutput copyNextSampleBuffer] + 68 (AVFoundation + 647318) [0x7fff40e75096]
                                          312  -[AVAssetReaderOutput copyNextSampleBuffer] + 151 (AVFoundation + 102837) [0x7fff40df01b5]
                                            312  ??? (MediaToolbox + 1877100) [0x7fff48f2b46c]
                                              312  FigSemaphoreWaitRelative + 149 (CoreMedia + 15095) [0x7fff45c35af7]
                                                312  WaitOnCondition + 11 (CoreMedia + 15418) [0x7fff45c35c3a]
                                                  312  __psynch_cvwait + 10 (libsystem_kernel.dylib + 117270) [0x7fff6cd06a16]
                                                   *312  psynch_cvcontinue + 0 (pthread + 38948) [0xffffff7f80f0d824]

This is with the latest Safari Technology Preview on macOS High Sierra.

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