Messages in this thread | | | Subject | Re: PATCH: Further aacraid work | From | James Bottomley <> | Date | 17 Jun 2004 11:37:04 -0500 |
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On Thu, 2004-06-17 at 11:32, Clay Haapala wrote: > Today's scatterlists already handle entries with a size greater than > MMU pagelength, even on x86, right? We have seen it in iSCSI driver > testing, though it is not the usual case. crypto/digests.c:update() > function was recently patched to handle the case and properly kmap() > the additional memory represented by the sg entry.
Yes. The bio layer does two operations on sg elements: clustering and virtual merging. Clustering tries to coalesce two adjacent I/O pages that also happen to be adjacent in memory physical space. virtual merging uses an IOMMU to place non-physically-adjacent pages at adjacent locations in bus physical space.
> So, on regular x86 this is a matter of convenience/timing, and the > page assignments will tend toward, but not always be, random 1-page > entries as the system is used.
That's what the jury is out on, I think. The question is could we improve the chances of clustering by altering the way the vm allocator works.
James
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