The data in bug 186422 show that the StringImpl heap uses about 20MB of dirty memory, but only contains about 6MB of data, so there's a potential memory saving here of up to 14MB. VIRTUAL RESIDENT DIRTY SWAPPED ALLOCATION BYTES DIRTY+SWAP REGION MALLOC ZONE SIZE SIZE SIZE SIZE COUNT ALLOCATED FRAG SIZE % FRAG COUNT =========== ======= ========= ========= ========= ========= ========= ========= ====== ====== WebKit StringImpl_0x10b4c0000 100.0M 23.2M 19.9M 0K 74493 6166K 13.9M 70% 27