8155e5d561
Specifically, teach pmap_qenter() to recognize the case when it is being asked to replace a mapping with the very same mapping and not generate a shootdown. Unfortunately, the buffer cache commonly passes an entire buffer to pmap_qenter() when only a subset of the mappings are changing. For the extension of buffers in allocbuf() this was resulting in unnecessary shootdowns. The addition of new pages to the end of the buffer need not and did not trigger a shootdown, but overwriting the initial mappings with the very same mappings was seen as a change that necessitated a shootdown. With this change, that is no longer so. For a "buildworld" on amd64, this change eliminates 14-15% of the pmap_invalidate_range() shootdowns, and about 4% of the overall shootdowns. MFC after: 3 weeks |
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