Re: [LSF/MM TOPIC]swap improvements for fast SSD

[Shaohua Li <shli@kernel.org>]

On Wed, Jan 23, 2013 at 03:05:22PM -0800, Dan Magenheimer wrote:
> I would be very interested in this topic.
> 
> > Because of high density, low power and low price, flash storage (SSD) is a good
> > candidate to partially replace DRAM. A quick answer for this is using SSD as
> > swap. But Linux swap is designed for slow hard disk storage. There are a lot of
> > challenges to efficiently use SSD for swap:
> > 
> > 1. Lock contentions (swap_lock, anon_vma mutex, swap address space lock)
> > 2. TLB flush overhead. To reclaim one page, we need at least 2 TLB flush. This
> > overhead is very high even in a normal 2-socket machine.
> > 3. Better swap IO pattern. Both direct and kswapd page reclaim can do swap,
> > which makes swap IO pattern is interleave. Block layer isn't always efficient
> > to do request merge. Such IO pattern also makes swap prefetch hard.
> 
> Shaohua --
> 
> Have you considered the possibility of subverting the block layer entirely
> and accessing the SSD like slow RAM rather than a fast I/O device?  E.g.
> something like NVME and as in this paper?
> 
> http://static.usenix.org/events/fast12/tech/full_papers/Yang.pdf 
> 
> If you think this could be an option, it could make a very
> interesting backend to frontswap (something like ramster).

We had discussion about this before, but looks this requires very low latency
storage, didn't take it serious yet.

Thanks,
Shaohua

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