On 16.09.19 03:41, Wei Wang wrote: > On 09/14/2019 02:36 AM, Tyler Sanderson wrote: >> Hello, I'm curious about the intent of VIRTIO_BALLOON_F_FREE_PAGE_HINT >> (commit >> <https://github.com/torvalds/linux/commit/86a559787e6f5cf662c081363f64a20cad654195#diff-fd202acf694d9eba19c8c64da3e480c9>). >> >> >> >> My understanding is that this mechanism works similarly to the >> existing inflate/deflate queues. Pages are allocated by the guest and >> then reported on VQ_FREE_PAGE. >> >> Question: Is there a limit to how many pages will be allocated? What >> controls the amount of memory pressure applied? > > No control for the limit currently. The implementation reports all the > guest free pages to host. > The main usage for this feature so far is to have guest skip sending > those guest free pages > (the more, the better) during live migration. > > >> >> In my experience with virtio balloon there are problems with the >> mechanisms that are supposed to deflate the balloon in response to >> memory pressure (e.g. OOM notifier). > > What problem did you see? We've also changed balloon to use memory shrinker, > did you see the problem with shrinker as well? > >> >> It seems an ideal balloon interface would allow the guest to round >> robin through free guest physical pages, allowing the host to unback >> them, but never having more than a few pages allocated to the balloon >> at any one time. For example: >> 1. Guest allocates 1 page and notifies balloon device of this page's >> address. >> 2. Host debacks the received page. >> 3. Guest frees the page. >> 4. Repeat at #1, but ensure that different pages are allocated each time. > > Probably you need a mechanism to "ensure" different pages to be allocated. > The current implementation (having balloon hold the allocated pages) could > be thought of as one mechanism (it is simple). > >> >> This way the "balloon size" is never more than a few pages and does >> not create memory pressure. However the difficulty is in ensuring each >> set of sent pages is disjoint from previously sent pages. Is there a >> mechanism to round-robin allocations through all of guest physical >> memory? Does VIRTIO_BALLOON_F_FREE_PAGE_HINT enable this?
There are use cases where you really want memory pressure (page cache is the prime example). Anyhow, I think the use case you want the "round-robin allocations" for is better tackled by "free page reporting" (used to be called "free page hinting") currently discussed on various lists. "allowing the host to unback them, but never having more than a few pages allocated to the balloon at any one time." is similar to what "free page reporting" does. We decided to only report bigger pages (avoid splitting up THP in the hypervisor, overhead) and only temporarily pull out a fixed amount of pages (16) from the page allocator to avoid false-OOM. Guaranteeing forward progress (similar to what you describe) is one important key concept. -- Thanks, David / dhildenb _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
