| commit | f1ca2511d0efc109db6fe1d87e9788918855ccad | |
| author | Edwin Smith <smith@fb.com> | |
| Thu, 21 Sep 2017 13:55:25 +0000 (21 06:55 -0700) | ||
| committer | Hhvm Bot <hhvm-bot@users.noreply.github.com> | |
| Thu, 21 Sep 2017 14:02:21 +0000 (21 07:02 -0700) | ||
| tree | 3824820140d23027849399ed71643fe2618b5663 | treesnapshot (tar.gz zip) |
| parent | f34e265447768031edc939dac57f38520e48a37e | commitdiff |
Clear mark state implicitly by incrementing a 2-bit version
Summary:
The GC init phase currently scans the whole heap, clearing the
mark bit for every object. By using a two-bit version, the
clear becomes O(1) by incrementing a global "version". Any object
with a different version is "unmarked".
This uses 2 bits instead of 1, so newly allocated objects can
hardcode the version value 0, instead of loading a global value.
The actual global version counter cycles between 1-2-3.
Implicit clearing of mark-state eliminates a lot of stores, but
doesn't eliminate the whole-heap scan yet; we still need it to
reconstruct crossing maps. That will be optimized separately.
Reviewed By: ricklavoie
Differential Revision: D5827657
fbshipit-source-id: bbba9666275c1e1386ee5231f713fe3b01b304af
Summary:
The GC init phase currently scans the whole heap, clearing the
mark bit for every object. By using a two-bit version, the
clear becomes O(1) by incrementing a global "version". Any object
with a different version is "unmarked".
This uses 2 bits instead of 1, so newly allocated objects can
hardcode the version value 0, instead of loading a global value.
The actual global version counter cycles between 1-2-3.
Implicit clearing of mark-state eliminates a lot of stores, but
doesn't eliminate the whole-heap scan yet; we still need it to
reconstruct crossing maps. That will be optimized separately.
Reviewed By: ricklavoie
Differential Revision: D5827657
fbshipit-source-id: bbba9666275c1e1386ee5231f713fe3b01b304af