Messages in this thread | | | Date | Thu, 16 Aug 2001 19:44:04 +0100 (BST) | From | Mark Hemment <> | Subject | Re: [PATCH] Align VM locks |
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On Thu, 16 Aug 2001, Andrea Arcangeli wrote: > BTW, for your other patch you sent a few hours ago you forgot to drop > the KMAP entry that is wasting NR_CPUS*PAGE_SIZE of virtual address > space: > > ftp://ftp.us.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.8aa1/00_create_bounces-sleeps-1
Eh, no I didn't.
At least on the work load I'm interest in, SpecFS v2.0 over NFSv3, removing the KM_BOUNCE_WRITE results in a performance drop (confirmed today).
It is often the case that when it comes time to write a page out it has lost any mapping it had when it was made dirty via a write(), so there is no side benefit of using a straight kmap().
By having KM_BOUNCE_WRITE we don't run through the "normal" mapping space on I/O. Not having KM_BOUNCE_WRITE causing extra shootdowns, which _are_ expensive, as the code needs to busy-wait for all the other engines (while the kmap_lock held - and on a 4-way there is a good chance one of the processors is running with interrupts disabled). KM_BOUNCE_WRITE may waste virtual address-space, but it saves on expensive shootdowns.
Mark
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