[tor-bugs] #29120 [Applications/Tor Browser]: Default value of media.cache_size (0) causes some media to load extremely slowly or become unplayable

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#29120: Default value of media.cache_size (0) causes some media to load extremely
slowly or become unplayable
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 Reporter:  QZw2aBQoPyuEVXYVlBps                 |          Owner:  tbb-
                                                 |  team
     Type:  defect                               |         Status:  new
 Priority:  High                                 |      Milestone:
Component:  Applications/Tor Browser             |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  tbb-disk-leak, tbb-usability-        |  Actual Points:
  website, TorBrowserTeam201901                  |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:
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Comment (by QZw2aBQoPyuEVXYVlBps):

 Replying to [comment:5 gk]:
 > Thanks for the patch! I wonder what happens if the video is larger than
 the available memory cache (or the latter is already filled up with other
 cached stuff). Does the browser crash then? Or become unusable otherwise?
 I tested using the video in the main ticket post, which is ~30mb (the
 MemoryCache was hardcoded to 8192000 bytes in the patch).

 As far as I can tell, it ''seems'' to properly reuse old memory once it
 hits the cache limit, I could play through the whole video without
 anything unusual happening, if I seeked back to the beginning after
 playing through it, it would make a new HTTP request to fetch that data,
 so it seemed to have properly fallen out of the cache.

 One thing that should be looked into is if the MemoryBlockCache can safely
 be used concurrently by multiple streams (as a "single global cache" like
 the FileBlockCache is), if you look at the code here:
 https://hg.mozilla.org/releases/mozilla-
 esr60/file/256453759958ed9c2eb17a0764d2fcfd7f8e3323/dom/media/MediaCache.cpp#l737

 You'll see that normally, a new MemoryBlockCache object is created for
 each resource below {{{media.memory_cache_max_size}}}, they don't seem to
 be reused across multiple resources like the FileBlockCache is.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/29120#comment:7>
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