44 lines
2.2 KiB
Plaintext
44 lines
2.2 KiB
Plaintext
$NetBSD: Pmap.notes,v 1.3 2000/03/13 23:52:28 soren Exp $
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Following are some observations about the BSD hp300 pmap module that
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may prove useful for other pmap modules:
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1. pmap_remove should be efficient with large, sparsely populated ranges.
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Profiling of exec/exit intensive work loads showed that much time was
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being spent in pmap_remove. This was primarily due to calls from exec
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when deallocating the stack segment. Since the current implementation
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of the stack is to "lazy allocate" the maximum possible stack size
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(typically 16-32mb) when the process is created, pmap_remove will be
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called with a large chunk of largely empty address space. It is
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important that this routine be able to quickly skip over large chunks
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of allocated but unpopulated VA space. The hp300 pmap module did check
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for unpopulated "segments" (which map 4mb chunks) and skipped them fairly
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efficiently but once it found a valid segment descriptor (STE), it rather
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clumsily moved forward over the PTEs mapping that segment. Particularly
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bad was that for every PTE it would recheck that the STE was valid even
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though we should already know that.
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pmap_protect can benefit from similar optimizations though it is
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(currently) not called with large regions.
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Another solution would be to change the way stack allocation is done
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(i.e. don't preallocate the entire address range) but I think it is
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important to be able to efficiently support such large, spare ranges
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that might show up in other applications (e.g. a randomly accessed
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large mapped file).
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2. Bit operations (i.e. ~,&,|) are more efficient than bitfields.
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This is a 68k/gcc issue, but if you are trying to squeeze out maximum
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performance...
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3. Don't flush TLB/caches for inactive mappings.
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On the hp300 the TLBs are either designed as, or used in such a way that,
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they are flushed on every context switch (i.e. there are no "process
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tags") Hence, doing TLB flushes on mappings that aren't associated with
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either the kernel or the currently running process are a waste. Seems
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pretty obvious but I missed it for many years. An analogous argument
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applies to flushing untagged virtually addressed caches (ala the 320/350).
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