Re: [RFC] transcendent memory for Linux

Pavel Machek
2009-06-27T11:28:55Z

Hi!

This description (whole mail) needs to go into Documentation/, somewhere.

> Normal memory is directly addressable by the kernel,
> of a known normally-fixed size, synchronously accessible,
> and persistent (though not across a reboot).
...
> Transcendent memory, or "tmem" for short, provides a
> well-defined API to access this unusual class of memory.
> The basic operations are page-copy-based and use a flexible
> object-oriented addressing mechanism. Tmem assumes

Should this API be documented, somewhere? Is it in-kernel API or does
userland see it?

> "Preswap" IS persistent, but for various reasons may not always
> be available for use, again due to factors that may not be
> visible to the kernel (but, briefly, if the kernel is being
> "good" and has shared its resources nicely, then it will be
> able to use preswap, else it will not). Once a page is put,
> a get on the page will always succeed. So when the kernel
> finds itself in a situation where it needs to swap out a page,
> it first attempts to use preswap. If the put works, a disk
> write and (usually) a disk read are avoided. If it doesn't,
> the page is written to swap as usual. Unlike precache, whether

Ok, how much slower this gets in the worst case? Single hypercall to
find out that preswap is unavailable? I guess that compared to disk
access that's lost in the noise?
Pavel

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