Which Scheme implementations currently have concurrent GC?
If my reading of the current manual is correct, Chez doesn't have it.
Ypsilon "implements 'mostly concurrent garbage collection', which is optimized for the multi-core CPU system". The website doesn't say which precise techniques it is using, but the source is available. http://www.littlewingpinball.net/mediawiki/index.php/Ypsilon_Scheme_System