65437 2011-07-31 15:10:24 -0700 JSC GC lays out size classes under wrong assumptions about expected object size. 2011-08-01 02:49:27 -0700 1 1 1 Unclassified WebKit JavaScriptCore 528+ (Nightly build) All All RESOLVED FIXED P2 Normal --- 65458 1 fpizlo webkit-unassigned fpizlo oliver webkit.review.bot oldest_to_newest 444461 0 fpizlo 2011-07-31 15:10:24 -0700 The typical object allocation in the GC heap is either 56 bytes (strings) or 64 bytes (objects) in 64-bit systems. Few allocations are ever smaller than this, and it's unlikely that an allocation would ever be less than 4 words, since every JSCell has two words (vtable and structure), leaving only two words for whatever payload the object may have. The current GC strategy lays out size classes starting at 8 bytes (the size of a double), corresponding to an impossible allocation size on 64-bit platforms and a highly unlikely one on 32-bit platforms. This has a number of undesirable outcomes. First, there are more mark bits in a typical MarkedBlock than necessary, leading to both wasted space and decreased locality when accessing mark bits. Second, it means that almost identical allocation sizes (56 bytes and 64 bytes for instance) require the creation of entirely separate MarkedBlocks. This may decrease internal fragmentation by as little (1/8th to be precise), but it will lead to higher external fragmentation: for example a string allocation will never be able to reuse space left behind by an object allocation, unless the object-sized MarkedBlock becomes entirely free, and we return it to the free block pool. The JSC GC should probably space out size classes more widely and start with a larger size class that is more representative of the smallest common allocation size. 444462 1 102466 fpizlo 2011-07-31 15:13:51 -0700 Created attachment 102466 the patch 444463 2 fpizlo 2011-07-31 15:19:24 -0700 This is a 1% win on both SunSpider and V8. 444473 3 102466 webkit.review.bot 2011-07-31 16:48:43 -0700 Comment on attachment 102466 the patch Clearing flags on attachment: 102466 Committed r92092: <http://trac.webkit.org/changeset/92092> 444474 4 webkit.review.bot 2011-07-31 16:48:47 -0700 All reviewed patches have been landed. Closing bug. 102466 2011-07-31 15:13:51 -0700 2011-07-31 16:48:43 -0700 the patch biggersizes_patch_1.diff text/plain 1726 fpizlo SW5kZXg6IFNvdXJjZS9KYXZhU2NyaXB0Q29yZS9DaGFuZ2VMb2cKPT09PT09PT09PT09PT09PT09 PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PQotLS0gU291 cmNlL0phdmFTY3JpcHRDb3JlL0NoYW5nZUxvZwkocmV2aXNpb24gOTIwOTApCisrKyBTb3VyY2Uv SmF2YVNjcmlwdENvcmUvQ2hhbmdlTG9nCSh3b3JraW5nIGNvcHkpCkBAIC0xLDMgKzEsMTcgQEAK KzIwMTEtMDctMzEgIEZpbGlwIFBpemxvICA8ZnBpemxvQGFwcGxlLmNvbT4KKworICAgICAgICBK U0MgR0MgbGF5cyBvdXQgc2l6ZSBjbGFzc2VzIHVuZGVyIHdyb25nIGFzc3VtcHRpb25zIGFib3V0 IGV4cGVjdGVkCisgICAgICAgIG9iamVjdCBzaXplLgorICAgICAgICBodHRwczovL2J1Z3Mud2Vi a2l0Lm9yZy9zaG93X2J1Zy5jZ2k/aWQ9NjU0MzcKKworICAgICAgICBSZXZpZXdlZCBieSBOT0JP RFkgKE9PUFMhKS4KKyAgICAgICAgCisgICAgICAgIENoYW5nZWQgdGhlIGF0b20gc2l6ZSAtIHdo aWNoIGlzIGJvdGggdGhlIHNtYWxsZXN0IGFsbG9jYXRpb24gc2l6ZSBhbmQKKyAgICAgICAgdGhl IHNtYWxsZXN0IHBvc3NpYmxlIHN0ZXBwaW5nIHVuaXQgZm9yIHNpemUgY2xhc3Mgc3BhY2luZyAt IGZyb20KKyAgICAgICAgOCBieXRlcyB0byA0IHBvaW50ZXItc2l6ZSB3b3Jkcy4gIFRoaXMgaXMg YSAxJSB3aW4gb24gU3VuU3BpZGVyLgorCisgICAgICAgICogaGVhcC9NYXJrZWRCbG9jay5oOgor CiAyMDExLTA3LTMxICBGaWxpcCBQaXpsbyAgPGZwaXpsb0BhcHBsZS5jb20+CiAKICAgICAgICAg REZHIG5vbi1zcGVjdWxhdGl2ZSBKSVQgZG9lcyBub3Qgb3B0aW1pemUgUHV0QnlWYWwKSW5kZXg6 IFNvdXJjZS9KYXZhU2NyaXB0Q29yZS9oZWFwL01hcmtlZEJsb2NrLmgKPT09PT09PT09PT09PT09 PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PQotLS0g U291cmNlL0phdmFTY3JpcHRDb3JlL2hlYXAvTWFya2VkQmxvY2suaAkocmV2aXNpb24gOTIwODUp CisrKyBTb3VyY2UvSmF2YVNjcmlwdENvcmUvaGVhcC9NYXJrZWRCbG9jay5oCSh3b3JraW5nIGNv cHkpCkBAIC00Miw3ICs0Miw5IEBAIG5hbWVzcGFjZSBKU0MgewogICAgIGNsYXNzIE1hcmtlZEJs b2NrIDogcHVibGljIERvdWJseUxpbmtlZExpc3ROb2RlPE1hcmtlZEJsb2NrPiB7CiAgICAgICAg IGZyaWVuZCBjbGFzcyBXVEY6OkRvdWJseUxpbmtlZExpc3ROb2RlPE1hcmtlZEJsb2NrPjsKICAg ICBwdWJsaWM6Ci0gICAgICAgIHN0YXRpYyBjb25zdCBzaXplX3QgYXRvbVNpemUgPSBzaXplb2Yo ZG91YmxlKTsgLy8gRW5zdXJlcyBuYXR1cmFsIGFsaWdubWVudCBmb3IgYWxsIGJ1aWx0LWluIHR5 cGVzLgorICAgICAgICAvLyBFbnN1cmUgbmF0dXJhbCBhbGlnbm1lbnQgZm9yIG5hdGl2ZSB0eXBl cyB3aGlsc3QgcmVjb2duaXppbmcgdGhhdCB0aGUgc21hbGxlc3QKKyAgICAgICAgLy8gb2JqZWN0 IHRoZSBoZWFwIHdpbGwgY29tbW9ubHkgYWxsb2NhdGUgaXMgZm91ciB3b3Jkcy4KKyAgICAgICAg c3RhdGljIGNvbnN0IHNpemVfdCBhdG9tU2l6ZSA9IDQgKiBzaXplb2Yodm9pZCopOwogICAgICAg ICBzdGF0aWMgY29uc3Qgc2l6ZV90IGJsb2NrU2l6ZSA9IDE2ICogS0I7CiAKICAgICAgICAgc3Rh dGljIGNvbnN0IHNpemVfdCBhdG9tc1BlckJsb2NrID0gYmxvY2tTaXplIC8gYXRvbVNpemU7IC8v IH4xLjUlIG92ZXJoZWFkCg==