> On 11/06/2016 07:37 PM, Li, Liang Z wrote: > >> Let's say we do a 32k bitmap that can hold ~1M pages. That's 4GB of RAM. > >> On a 1TB system, that's 256 passes through the top-level loop. > >> The bottom-level lists have tens of thousands of pages in them, even > >> on my laptop. Only 1/256 of these pages will get consumed in a given pass. > >> > > Your description is not exactly. > > A 32k bitmap is used only when there is few free memory left in the > > system and when the extend_page_bitmap() failed to allocate more > > memory for the bitmap. Or dozens of 32k split bitmap will be used, > > this version limit the bitmap count to 32, it means we can use at most > > 32*32 kB for the bitmap, which can cover 128GB for RAM. We can increase > the bitmap count limit to a larger value if 32 is not big enough. > > OK, so it tries to allocate a large bitmap. But, if it fails, it will try to work with a > smaller bitmap. Correct? > Yes. > So, what's the _worst_ case? It sounds like it is even worse than I was > positing. > Only a 32KB bitmap can be allocated, and there are a huge amount of low order (<3) free pages is the worst case. > >> That's an awfully inefficient way of doing it. This patch > >> essentially changed the data structure without changing the algorithm to > populate it. > >> > >> Please change the *algorithm* to use the new data structure efficiently. > >> Such a change would only do a single pass through each freelist, and > >> would choose whether to use the extent-based (pfn -> range) or > >> bitmap-based approach based on the contents of the free lists. > > > > Save the free page info to a raw bitmap first and then process the raw > > bitmap to get the proper ' extent-based ' and 'bitmap-based' is the > > most efficient way I can come up with to save the virtio data transmission. > Do you have some better idea? > > That's kinda my point. This patch *does* processing to try to pack the > bitmaps full of pages from the various pfn ranges. It's a form of processing > that gets *REALLY*, *REALLY* bad in some (admittedly obscure) cases. > > Let's not pretend that making an essentially unlimited number of passes over > the free lists is not processing. > > 1. Allocate as large of a bitmap as you can. (what you already do) 2. Iterate > from the largest freelist order. Store those pages in the > bitmap. > 3. If you can no longer fit pages in the bitmap, return the list that > you have. > 4. Make an approximation about where the bitmap does not make any more, > and fall back to listing individual PFNs. This would make sens, for > instance in a large zone with very few free order-0 pages left. > Sounds good. Should we ignore some of the order-0 pages in step 4 if the bitmap is full? Or should retry to get a complete list of order-0 pages? > > > It seems the benefit we get for this feature is not as big as that in fast > balloon inflating/deflating. > >> > >> You should not be using get_max_pfn(). Any patch set that continues > >> to use it is not likely to be using a proper algorithm. > > > > Do you have any suggestion about how to avoid it? > > Yes: get the pfns from the page free lists alone. Don't derive them from the > pfn limits of the system or zones. The ' get_max_pfn()' can be avoid in this patch, but I think we can't avoid it completely. We need it as a hint for allocating a proper size bitmap. No? Thanks! Liang _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
