Messages in this thread | | | Date | Mon, 26 Apr 2010 09:01:51 +0300 | From | Avi Kivity <> | Subject | Re: Frontswap [PATCH 0/4] (was Transcendent Memory): overview |
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On 04/25/2010 06:29 PM, Dan Magenheimer wrote: >>> 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. >> > When pages are 2MB, this may be true. When pages are 4KB and > copied individually, it may take longer to program a DMA engine > than to just copy 4KB. >
Of course, you have to use a batching API, like virtio or Xen's rings, to avoid the overhead.
> But in any case, frontswap works fine on all existing machines > today. If/when most commodity CPUs have an asynchronous RAM DMA > engine, an asynchronous API may be appropriate. Or the existing > swap API might be appropriate. Or the synchronous frontswap API > may work fine too. Speculating further about non-existent > hardware that might exist in the (possibly far) future is irrelevant > to the proposed patch, which works today on all existing x86 hardware > and on shipping software. >
dma engines are present on commodity hardware now:
http://en.wikipedia.org/wiki/I/O_Acceleration_Technology
I don't know if consumer machines have them, but servers certainly do. modprobe ioatdma.
-- Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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