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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCH 3 of 3 V3] tools/libxc: Document checkpoint compression in xg_save_restore.h
# HG changeset patch # User Shriram Rajagopalan <rshriram@xxxxxxxxx> # Date 1318545418 25200 # Node ID 37fa1bdbecacd8fefcc382183ad25fd6c76300c3 # Parent 32c4d844813aa22ca05df0b855857a607ac4c056 tools/libxc: Document checkpoint compression in xg_save_restore.h Add comments to xg_save_restore.h explaining changes in Remus wire protocol when checkpoint compression is enabled. Signed-off-by: Shriram Rajagopalan <rshriram@xxxxxxxxx> diff -r 32c4d844813a -r 37fa1bdbecac tools/libxc/xg_save_restore.h --- a/tools/libxc/xg_save_restore.h Thu Oct 13 15:36:55 2011 -0700 +++ b/tools/libxc/xg_save_restore.h Thu Oct 13 15:36:58 2011 -0700 @@ -67,7 +67,7 @@ * * consists of p2m_size bytes comprising an array of xen_pfn_t sized entries. * - * BODY PHASE + * BODY PHASE - Format A (for live migration or Remus without compression) * ---------- * * A series of chunks with a common header: @@ -87,6 +87,122 @@ * * If chunk type is 0 then body phase is complete. * + * + * BODY PHASE - Format B (for Remus with compression) + * ---------- + * + * A series of chunks with a common header: + * int : chunk type + * + * If the chunk type is +ve then chunk contains array of PFNs corresponding + * to guest memory and type contains the number of PFNs in the batch: + * + * unsigned long[] : PFN array, length == number of pages in batch + * Each entry consists of XEN_DOMCTL_PFINFO_* + * in bits 31-28 and the PFN number in bits 27-0. + * + * If the chunk type is -ve then chunk consists of one of a number of + * metadata types. See definitions of XC_SAVE_ID_* below. + * + * If the chunk type is -ve and equals XC_SAVE_ID_COMPRESSED_DATA, then the + * chunk consists of compressed page data, in the following format: + * + * unsigned long : Size of the compressed chunk to follow + * compressed data : variable length data of size indicated above. + * This chunk consists of compressed page data. + * The number of pages in one chunk depends on + * the amount of space available in the sender's + * output buffer. + * + * Format of compressed data: + * compressed_data = <deltas>* + * delta = <marker, run*> + * marker = (RUNFLAG|SKIPFLAG) bitwise-or RUNLEN [1 byte marker] + * RUNFLAG = 0 + * SKIPFLAG = 1 << 7 + * RUNLEN = 7-bit unsigned value indicating number of WORDS in the run + * run = string of bytes of length sizeof(WORD) * RUNLEN + * + * If marker contains RUNFLAG, then RUNLEN * sizeof(WORD) bytes of data following + * the marker is copied into the target page at the appropriate offset indicated by + * the offset_ptr + * If marker contains SKIPFLAG, then the offset_ptr is advanced + * by RUNLEN * sizeof(WORD). + * + * If chunk type is 0 then body phase is complete. + * + * There can be one or more chunks with type XC_SAVE_ID_COMPRESSED_DATA, + * containing compressed pages. The compressed chunks are collated to form + * one single compressed chunk for the entire iteration. The number of pages + * present in this final compressed chunk will be equal to the total number + * of valid PFNs specified by the +ve chunks. + * + * At the sender side, compressed pages are inserted into the output stream + * in the same order as they would have been if compression logic was absent. + * + * Until last iteration, the BODY is sent in Format A, to maintain live + * migration compatibility with receivers of older Xen versions. + * At the last iteration, if Remus compression was enabled, the sender sends + * a trigger, XC_SAVE_ID_ENABLE_COMPRESSION to tell the receiver to parse the + * BODY in Format B from the next iteration onwards. + * + * An example sequence of chunks received in Format B: + * +16 +ve chunk + * unsigned long[16] PFN array + * +100 +ve chunk + * unsigned long[100] PFN array + * +50 +ve chunk + * unsigned long[50] PFN array + * + * XC_SAVE_ID_COMPRESSED_DATA TAG + * N Length of compressed data + * N bytes of DATA Decompresses to 166 pages + * + * XC_SAVE_ID_* other xc save chunks + * 0 END BODY TAG + * + * Corner case with checkpoint compression: + * At sender side, after pausing the domain, dirty pages are usually + * copied out to a temporary buffer. After the domain is resumed, + * compression is done and the compressed chunk(s) are sent, followed by + * other XC_SAVE_ID_* chunks. + * If the temporary buffer gets full while scanning for dirty pages, + * the sender stops buffering of dirty pages, compresses the temporary + * buffer and sends the compressed data with XC_SAVE_ID_COMPRESSED_DATA. + * The sender then resumes the buffering of dirty pages and continues + * scanning for the dirty pages. + * For e.g., assume that the temporary buffer can hold 4096 pages and + * there are 5000 dirty pages. The following is the sequence of chunks + * that the receiver will see: + * + * +1024 +ve chunk + * unsigned long[1024] PFN array + * +1024 +ve chunk + * unsigned long[1024] PFN array + * +1024 +ve chunk + * unsigned long[1024] PFN array + * +1024 +ve chunk + * unsigned long[1024] PFN array + * + * XC_SAVE_ID_COMPRESSED_DATA TAG + * N Length of compressed data + * N bytes of DATA Decompresses to 4096 pages + * + * +4 +ve chunk + * unsigned long[4] PFN array + * + * XC_SAVE_ID_COMPRESSED_DATA TAG + * M Length of compressed data + * M bytes of DATA Decompresses to 4 pages + * + * XC_SAVE_ID_* other xc save chunks + * 0 END BODY TAG + * + * In other words, XC_SAVE_ID_COMPRESSED_DATA can be interleaved with + * +ve chunks arbitrarily. But at the receiver end, the following condition + * always holds true until the end of BODY PHASE: + * num(PFN entries +ve chunks) >= num(pages received in compressed form) + * * TAIL PHASE * ---------- * _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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