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diff --git a/qemu/docs/xbzrle.txt b/qemu/docs/xbzrle.txt deleted file mode 100644 index 52c8511a4..000000000 --- a/qemu/docs/xbzrle.txt +++ /dev/null @@ -1,136 +0,0 @@ -XBZRLE (Xor Based Zero Run Length Encoding) -=========================================== - -Using XBZRLE (Xor Based Zero Run Length Encoding) allows for the reduction -of VM downtime and the total live-migration time of Virtual machines. -It is particularly useful for virtual machines running memory write intensive -workloads that are typical of large enterprise applications such as SAP ERP -Systems, and generally speaking for any application that uses a sparse memory -update pattern. - -Instead of sending the changed guest memory page this solution will send a -compressed version of the updates, thus reducing the amount of data sent during -live migration. -In order to be able to calculate the update, the previous memory pages need to -be stored on the source. Those pages are stored in a dedicated cache -(hash table) and are accessed by their address. -The larger the cache size the better the chances are that the page has already -been stored in the cache. -A small cache size will result in high cache miss rate. -Cache size can be changed before and during migration. - -Format -======= - -The compression format performs a XOR between the previous and current content -of the page, where zero represents an unchanged value. -The page data delta is represented by zero and non zero runs. -A zero run is represented by its length (in bytes). -A non zero run is represented by its length (in bytes) and the new data. -The run length is encoded using ULEB128 (http://en.wikipedia.org/wiki/LEB128) - -There can be more than one valid encoding, the sender may send a longer encoding -for the benefit of reducing computation cost. - -page = zrun nzrun - | zrun nzrun page - -zrun = length - -nzrun = length byte... - -length = uleb128 encoded integer - -On the sender side XBZRLE is used as a compact delta encoding of page updates, -retrieving the old page content from the cache (default size of 512 MB). The -receiving side uses the existing page's content and XBZRLE to decode the new -page's content. - -This work was originally based on research results published -VEE 2011: Evaluation of Delta Compression Techniques for Efficient Live -Migration of Large Virtual Machines by Benoit, Svard, Tordsson and Elmroth. -Additionally the delta encoder XBRLE was improved further using the XBZRLE -instead. - -XBZRLE has a sustained bandwidth of 2-2.5 GB/s for typical workloads making it -ideal for in-line, real-time encoding such as is needed for live-migration. - -Example -old buffer: -1001 zeros -05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 68 00 00 6b 00 6d -3074 zeros - -new buffer: -1001 zeros -01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 68 00 00 67 00 69 -3074 zeros - -encoded buffer: - -encoded length 24 -e9 07 0f 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 03 01 67 01 01 69 - -Cache update strategy -===================== -Keeping the hot pages in the cache is effective for decreased cache -misses. XBZRLE uses a counter as the age of each page. The counter will -increase after each ram dirty bitmap sync. When a cache conflict is -detected, XBZRLE will only evict pages in the cache that are older than -a threshold. - -Usage -====================== -1. Verify the destination QEMU version is able to decode the new format. - {qemu} info migrate_capabilities - {qemu} xbzrle: off , ... - -2. Activate xbzrle on both source and destination: - {qemu} migrate_set_capability xbzrle on - -3. Set the XBZRLE cache size - the cache size is in MBytes and should be a -power of 2. The cache default value is 64MBytes. (on source only) - {qemu} migrate_set_cache_size 256m - -4. Start outgoing migration - {qemu} migrate -d tcp:destination.host:4444 - {qemu} info migrate - capabilities: xbzrle: on - Migration status: active - transferred ram: A kbytes - remaining ram: B kbytes - total ram: C kbytes - total time: D milliseconds - duplicate: E pages - normal: F pages - normal bytes: G kbytes - cache size: H bytes - xbzrle transferred: I kbytes - xbzrle pages: J pages - xbzrle cache miss: K - xbzrle overflow : L - -xbzrle cache-miss: the number of cache misses to date - high cache-miss rate -indicates that the cache size is set too low. -xbzrle overflow: the number of overflows in the decoding which where the delta -could not be compressed. This can happen if the changes in the pages are too -large or there are many short changes; for example, changing every second byte -(half a page). - -Testing: Testing indicated that live migration with XBZRLE was completed in 110 -seconds, whereas without it would not be able to complete. - -A simple synthetic memory r/w load generator: -.. include <stdlib.h> -.. include <stdio.h> -.. int main() -.. { -.. char *buf = (char *) calloc(4096, 4096); -.. while (1) { -.. int i; -.. for (i = 0; i < 4096 * 4; i++) { -.. buf[i * 4096 / 4]++; -.. } -.. printf("."); -.. } -.. } |