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). 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. - 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). > >> OTOH frontswap approach gets rid of any such artifacts and overheads. >> (ramzswap: http://code.google.com/p/compcache/) >> > > 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. 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. Thanks, Nitin -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>