Re: Frontswap [PATCH 0/4] (was Transcendent Memory): overview

[Avi Kivity]

On 04/25/2010 07:05 PM, Nitin Gupta wrote:
> > > Increasing the frequency of discards is also not an option:
> > >    - Creating discard bio requests themselves need memory and these
> > > swap devices
> > > come into picture only under low memory conditions.
> > >
> > >        
> > That's fine, swap works under low memory conditions by using reserves.
> >
> >      
> Ok, but still all this bio allocation and block layer overhead seems
> unnecessary and is easily avoidable. I think frontswap code needs
> clean up but at least it avoids all this bio overhead.
>    

Ok.  I agree it is silly to go through the block layer and end up 
servicing it within the kernel.

> > >    - We need to regularly scan swap_map to issue these discards.
> > > Increasing discard
> > > frequency also means more frequent scanning (which will still not be
> > > fast enough
> > > for ramzswap needs).
> > >
> > >        
> > How does frontswap do this?  Does it maintain its own data structures?
> >
> >      
> frontswap simply calls frontswap_flush_page() in swap_entry_free() i.e. as
> soon as a swap slot is freed. No bio allocation etc.
>    

The same code could also issue the discard?

> > Even for copying to RAM an async API is wanted, so you can dma it
> > instead of copying.
> >
> >      
> Maybe incremental development is better? Stabilize and refine existing
> code and gradually move to async API, if required in future?
>    

Incremental development is fine, especially for ramzswap where the APIs 
are all internal.  I'm more worried about external interfaces, these 
stick around a lot longer and if not done right they're a pain forever.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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