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Typo

Typo

Posted Dec 19, 2010 4:47 UTC (Sun) by quotemstr (subscriber, #45331)
In reply to: Typo by Halmonster
Parent article: Unmapped page cache control

Doesn't having the host use uncached IO to service virtualized guest IO eliminate the duplicate caching?


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Obvious Answer?

Posted Dec 19, 2010 6:42 UTC (Sun) by Fowl (subscriber, #65667) [Link]

Or just the disabling the cache on the guests? Thereby allowing them to all share the host's.

What am I missing?

Obvious Answer?

Posted Dec 19, 2010 7:07 UTC (Sun) by quotemstr (subscriber, #45331) [Link]

It's easier and less invasive to configure a program to use unbuffered IO for a particular program's access to a particular file (a virtual disk in this case) than to disable buffering for a whole operating system, which might be actually be impossible.

Typo

Posted Dec 20, 2010 11:39 UTC (Mon) by joern (subscriber, #22392) [Link]

> Doesn't having the host use uncached IO to service virtualized guest IO eliminate the duplicate caching?

It does - if only a single guest is caching the pages in question.

Interesting

Posted Dec 20, 2010 14:32 UTC (Mon) by rilder (guest, #59804) [Link]

I don't think that is possible -- to be able to differentiate requests from the host and the guest to serve uncached to guest and cached to host -- if this is possible then it is great.

Interesting

Posted Dec 20, 2010 14:39 UTC (Mon) by quotemstr (subscriber, #45331) [Link]

It's not only possible, but trivial: IO from guests goes through the hypervisor, which can use simple uncached file IO (e.g., O_DIRECT) to service these requests. Host IO goes through the normal path and uses normal buffering.

Interesting

Posted Jan 3, 2011 5:17 UTC (Mon) by balbir_singh (subscriber, #34142) [Link]

Yes, it is possible, but there have been reports of large overheads in doing so. Please see the email from Chris at http://www.mail-archive.com/kvm@vger.kernel.org/msg30821.... for throughput issues.


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