36673 2010-03-26 13:47:17 -0700 Math.random repeats values in different workers 2017-03-19 13:51:03 -0700 1 1 1 Unclassified WebKit JavaScriptCore 528+ (Nightly build) PC OS X 10.6 NEW P2 Normal --- 1 webkit webkit-unassigned ap bfulgham cben jiewen_tan oliver webkit oldest_to_newest 204767 0 webkit 2010-03-26 13:47:17 -0700 When calling Math.random() in different workers, Math.random() is returning the same values in the same order in each worker. For example, when executing the following code (in an object) in webkit in multiple workers, the object's redness is set to the same value in each worker: this.redness = Math.round(255*Math.random()); I do not have this problem in Chrome or in Firefox. If additional detail is required, the complete code is at the following URL: http://github.com/bennytheshap/sabmit/raw/master/sabmit/public/javascripts/turtlestuff/turtle_base.js 204887 1 oliver 2010-03-26 18:11:15 -0700 We seed the random number generator with WTF::currentTime at JSGlobalData.cpp:146 -- theoretically if we're spinning the new contexts up fast enough they'll all have the same seed. Should we seed with randomNumber() instead? I haven't tested to confirm this myself so this is simply theorising... 204904 2 webkit 2010-03-26 18:51:09 -0700 What's odd is that it happens even when the workers are created with a large amount of time between them. To see this in action, go here: http://evil.getdown.org:3000/breeds/1 Click the Hatch! button. It behaves as expected on Firefox and Chrome. 248710 3 oliver 2010-07-08 21:24:36 -0700 Just discovered that when we switched to our new random number generator, we reintroduced seeding from the current time, if the new workers are all initialiser sufficiently quickly, the generator gets a shared seed. 922831 4 cben 2013-08-30 00:12:06 -0700 Is this still relevant? I think your comments refer to https://trac.webkit.org/changeset/50789/trunk/JavaScriptCore/runtime/JSGlobalData.cpp but JSGlobalData.cpp has been renamed VM.cpp since and no longer includes any seeding. Seeding WeakRandom from time was fixed in Aug 2010 in https://trac.webkit.org/changeset/65947 . What I see now: Math.random() https://trac.webkit.org/browser/trunk/Source/JavaScriptCore/runtime/MathObject.cpp?rev=154868 calls exec->lexicalGlobalObject()->weakRandomNumber() implemented in https://trac.webkit.org/browser/trunk/Source/JavaScriptCore/runtime/JSGlobalObject.h?rev=154868#L490 as a shallow wrapper for https://trac.webkit.org/browser/trunk/Source/JavaScriptCore/runtime/WeakRandom.h It is initialized in https://trac.webkit.org/browser/trunk/Source/JavaScriptCore/runtime/JSGlobalObject.cpp?rev=154868#L138 from randomNumber(): https://trac.webkit.org/browser/trunk/Source/WTF/wtf/RandomNumber.cpp?rev=154868 which is a shallow wrapper for cryptographicallyRandomNumber() which is a shared ARC4Random state: https://trac.webkit.org/browser/trunk/Source/WTF/wtf/CryptographicallyRandomNumber.cpp?rev=154868#L168 which initializes itself in ARC4RandomNumberGenerator::stir() from strong OS randomness: https://trac.webkit.org/browser/trunk/Source/WTF/wtf/OSRandomSource.h?rev=154868 Phew. -- There is also https://trac.webkit.org/browser/trunk/Source/WTF/wtf/RandomNumberSeed.h?rev=154868 which *does* initialize on Unix from time & process id. But what it initializes are rand()/random() which are not used in any way for Math.random() AFAICT.