Messages in this thread | | | Date | Tue, 29 Mar 2005 20:23:56 +1000 | From | Nick Piggin <> | Subject | Re: [patch] use cheaper elv_queue_empty when unplug a device |
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Arjan van de Ven wrote: > On Tue, 2005-03-29 at 19:19 +1000, Nick Piggin wrote: > >>- removes the relock/retry merge mechanism in __make_request if we >> aren't able to get the GFP_ATOMIC allocation. Just fall through >> and assume the chances of getting a merge will be small (is this >> a valid assumption? Should measure it I guess). > > > this may have a potential problem; if the vm gets in trouble, you > suddenly start to generate worse IO patterns, which means IO performance > goes down right when it's most needed..... >
Sorry my wording was incorrect. It currently *always* retries the merge if it had at first failed, and after the patch, we never retry. So it should not result in behavioural shifts when there is a VM load is high.
It seems to be a clear source of problems for Kenneth though, because his workload appears to have almost zero merges, so he'll always be invoking the merge logic twice.
I agree there is potential for subtle interactions. But generally the block layer is surprisingly well behaved in my experience.
As Jens said, the complete removal of the GFP_ATOMIC allocation probably has the most potential for problems in this regard, although bios are not using GFP_ATOMIC allocations, so I would be a little surprised if it made a really noticable difference.
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