Messages in this thread | | | Date | Sun, 25 Apr 2010 17:15:16 +0300 | From | Avi Kivity <> | Subject | Re: Frontswap [PATCH 0/4] (was Transcendent Memory): overview |
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On 04/25/2010 04:37 PM, Dan Magenheimer wrote: >> My issue is with the API's synchronous nature. Both RAM and more >> exotic >> memories can be used with DMA instead of copying. A synchronous >> interface gives this up. >> : >> Let's not allow the urge to merge prevent us from doing the right >> thing. >> : >> I see. Given that swap-to-flash will soon be way more common than >> frontswap, it needs to be solved (either in flash or in the swap code). >> > While I admit that I started this whole discussion by implying > that frontswap (and cleancache) might be useful for SSDs, I think > we are going far astray here. Frontswap is synchronous for a > reason: It uses real RAM, but RAM that is not directly addressable > by a (guest) kernel. SSD's (at least today) are still I/O devices; > even though they may be very fast, they still live on a PCI (or > slower) bus and use DMA. Frontswap is not intended for use with > I/O devices. > > Today's memory technologies are either RAM that can be addressed > by the kernel, or I/O devices that sit on an I/O bus. The > exotic memories that I am referring to may be a hybrid: > memory that is fast enough to live on a QPI/hypertransport, > but slow enough that you wouldn't want to randomly mix and > hand out to userland apps some pages from "exotic RAM" and some > pages from "normal RAM". Such memory makes no sense today > because OS's wouldn't know what to do with it. But it MAY > make sense with frontswap (and cleancache). > > Nevertheless, frontswap works great today with a bare-metal > hypervisor. I think it stands on its own merits, regardless > of one's vision of future SSD/memory technologies. >
Even when frontswapping to RAM on a bare metal hypervisor it makes sense to use an async API, in case you have a DMA engine on board.
-- error compiling committee.c: too many arguments to function
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