Re: [GIT PULL] mm: frontswap (for 3.2 window)

Avi Kivity
2011-11-02T15:45:08Z

On 11/01/2011 12:16 PM, James Bottomley wrote:
> Actually, I think there's an unexpressed fifth requirement:
>
> 5. The optimised use case should be for non-paging situations.
>
> The problem here is that almost every data centre person tries very hard
> to make sure their systems never tip into the swap zone. A lot of
> hosting datacentres use tons of cgroup controllers for this and
> deliberately never configure swap which makes transcendent memory
> useless to them under the current API. I'm not sure this is fixable,
> but it's the reason why a large swathe of users would never be
> interested in the patches, because they by design never operate in the
> region transcended memory is currently looking to address.
>
> This isn't an inherent design flaw, but it does ask the question "is
> your design scope too narrow?"

If you look at cleancache, then it addresses this concern - it extends
pagecache through host memory. When dropping a page from the tail of
the LRU it first goes into tmem, and when reading in a page from disk
you first try to read it from tmem. However in many workloads,
cleancache is actually detrimental. If you have a lot of cache misses,
then every one of them causes a pointless vmexit; considering that
servers today can chew hundreds of megabytes per second, this adds up.
On the other side, if you have a use-once workload, then every page that
falls of the tail of the LRU causes a vmexit and a pointless page copy.

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