Messages in this thread | | | Date | Sun, 25 Apr 2010 15:16:26 +0300 | From | Avi Kivity <> | Subject | Re: Frontswap [PATCH 0/4] (was Transcendent Memory): overview |
| |
On 04/25/2010 06:11 AM, Nitin Gupta wrote: > On 04/24/2010 11:57 PM, Avi Kivity wrote: > >> On 04/24/2010 04:49 AM, Nitin Gupta wrote: >> >>> >>>> I see. So why not implement this as an ordinary swap device, with a >>>> higher priority than the disk device? this way we reuse an API and keep >>>> things asynchronous, instead of introducing a special purpose API. >>>> >>>> >>>> >>> ramzswap is exactly this: an ordinary swap device which stores every page >>> in (compressed) memory and its enabled as highest priority swap. >>> Currently, >>> it stores these compressed chunks in guest memory itself but it is not >>> very >>> difficult to send these chunks out to host/hypervisor using virtio. >>> >>> However, it suffers from unnecessary block I/O layer overhead and >>> requires >>> weird hooks in swap code, say to get notification when a swap slot is >>> freed. >>> >>> >> Isn't that TRIM? >> > No: trim or discard is not useful. The problem is that we require a callback > _as soon as_ a page (swap slot) is freed. Otherwise, stale data quickly accumulates > in memory defeating the whole purpose of in-memory compressed swap devices (like ramzswap). >
Doesn't flash have similar requirements? The earlier you discard, the likelier you are to reuse an erase block (or reduce the amount of copying).
> 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.
> - 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?
>> Maybe we should optimize these overheads instead. Swap used to always >> be to slow devices, but swap-to-flash has the potential to make swap act >> like an extension of RAM. >> >> > Spending lot of effort optimizing an overhead which can be completely avoided > is probably not worth it. >
I'm not sure. Swap-to-flash will soon be everywhere. If it's slow, people will feel it a lot more than ramzswap slowness.
> Also, I think the choice of a synchronous style API for frontswap and cleancache > is justified as they want to send pages to host *RAM*. If you want to use other > devices like SSDs, then these should be just added as another swap device as > we do currently -- these should not be used as frontswap storage directly. >
Even for copying to RAM an async API is wanted, so you can dma it instead of copying.
-- error compiling committee.c: too many arguments to function
| |