Hello, On Wed, Dec 07, 2016 at 08:57:01AM -0800, Dave Hansen wrote: > It is more space-efficient. We're fitting the order into 6 bits, which > would allows the full 2^64 address space to be represented in one entry, Very large order is the same as very large len, 6 bits of order or 8 bytes of len won't really move the needle here, simpler code is preferable. The main benefit of "len" is that it can be more granular, plus it's simpler than the bitmap too. Eventually all this stuff has to end up into a madvisev (not yet upstream but somebody posted it for jemalloc and should get merged eventually). So the bitmap shall be demuxed to a addr,len array anyway, the bitmap won't ever be sent to the madvise syscall, which makes the intermediate representation with the bitmap a complication with basically no benefits compared to a (N, [addr1,len1], .., [addrN, lenN]) representation. If you prefer 1 byte of order (not just 6 bits) instead 8bytes of len that's possible too, I wouldn't be against that, the conversion before calling madvise would be pretty efficient too. > and leaves room for the bitmap size to be encoded as well, if we decide > we need a bitmap in the future. How would a bitmap ever be useful with very large page-order? > If that was purely a length, we'd be limited to 64*4k pages per entry, > which isn't even a full large page. I don't follow here. What we suggest is to send the data down represented as (N, [addr1,len1], ..., [addrN, lenN]) which allows infinite ranges each one of maximum length 2^64, so 2^64 multiplied infinite times if you wish. Simplifying the code and not having any bitmap at all and no :6 :6 bits either. The high order to low order loop of allocations is the interesting part here, not the bitmap, and the fact of doing a single vmexit to send the large ranges. Once we pull out the largest order regions, we just add them to the array as [addr,1UL<<order], when the array reaches a maximum N number of entries or we fail a order 0 allocation, we flush all those entries down to qemu. Qemu then builds the iov for madvisev and it's pretty much a 1:1 conversion, not a decoding operation converting the bitmap in the (N, [addr1,len1], ..., [addrN, lenN]) for madvisev (or a flood of madvise MADV_DONTNEED with current kernels). Considering the loop that allocates starting from MAX_ORDER..1, the chance the bitmap is actually getting filled with more than one bit at page_shift of PAGE_SHIFT should be very low after some uptime. By the very nature of this loop, if we already exacerbates all high order buddies, the page-order 0 pages obtained are going to be fairly fragmented reducing the usefulness of the bitmap and potentially only wasting CPU/memory. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
