doc: Update multi-thread compression doc

Modify the doc to fit the previous changes.

Signed-off-by: Zeyu Jin <jinzeyu@huawei.com>
Signed-off-by: Ying Fang <fangying1@huawei.com>

doc-Update-multi-thread-compression-doc.patch

@@ -0,0 +1,85 @@
+From 642df85795097017e9370a9721f702cbec50c173 Mon Sep 17 00:00:00 2001
+From: Zeyu Jin <jinzeyu@huawei.com>
+Date: Sat, 30 Jan 2021 16:36:47 +0800
+Subject: [PATCH] doc: Update multi-thread compression doc
+
+Modify the doc to fit the previous changes.
+
+Signed-off-by: Zeyu Jin <jinzeyu@huawei.com>
+Signed-off-by: Ying Fang <fangying1@huawei.com>
+---
+ docs/multi-thread-compression.txt | 31 ++++++++++++++++++-------------
+ 1 file changed, 18 insertions(+), 13 deletions(-)
+
+diff --git a/docs/multi-thread-compression.txt b/docs/multi-thread-compression.txt
+index bb88c6bdf1..d429963cb0 100644
+--- a/docs/multi-thread-compression.txt
++++ b/docs/multi-thread-compression.txt
+@@ -33,14 +33,15 @@ thread compression can be used to accelerate the compression process.
+ 
+ The decompression speed of Zlib is at least 4 times as quick as
+ compression, if the source and destination CPU have equal speed,
+-keeping the compression thread count 4 times the decompression
+-thread count can avoid resource waste.
++and you choose Zlib as compression method, keeping the compression
++thread count 4 times the decompression thread count can avoid resource waste.
+ 
+ Compression level can be used to control the compression speed and the
+-compression ratio. High compression ratio will take more time, level 0
+-stands for no compression, level 1 stands for the best compression
+-speed, and level 9 stands for the best compression ratio. Users can
+-select a level number between 0 and 9.
++compression ratio. High compression ratio will take more time,
++level 1 stands for the best compression speed, and higher level means higher
++compression ration. For Zlib, users can select a level number between 0 and 9,
++where level 0 stands for no compression. For Zstd, users can select a
++level number between 1 and 22.
+ 
+ 
+ When to use the multiple thread compression in live migration
+@@ -116,16 +117,19 @@ to support the multiple thread compression migration:
+ 2. Activate compression on the source:
+     {qemu} migrate_set_capability compress on
+ 
+-3. Set the compression thread count on source:
++3. Set the compression method:
++    {qemu} migrate_set_parameter compress_method zstd
++
++4. Set the compression thread count on source:
+     {qemu} migrate_set_parameter compress_threads 12
+ 
+-4. Set the compression level on the source:
++5. Set the compression level on the source:
+     {qemu} migrate_set_parameter compress_level 1
+ 
+-5. Set the decompression thread count on destination:
++6. Set the decompression thread count on destination:
+     {qemu} migrate_set_parameter decompress_threads 3
+ 
+-6. Start outgoing migration:
++7. Start outgoing migration:
+     {qemu} migrate -d tcp:destination.host:4444
+     {qemu} info migrate
+     Capabilities: ... compress: on
+@@ -136,6 +140,7 @@ The following are the default settings:
+     compress_threads: 8
+     decompress_threads: 2
+     compress_level: 1 (which means best speed)
++    compress_method: zlib
+ 
+ So, only the first two steps are required to use the multiple
+ thread compression in migration. You can do more if the default
+@@ -143,7 +148,7 @@ settings are not appropriate.
+ 
+ TODO
+ ====
+-Some faster (de)compression method such as LZ4 and Quicklz can help
+-to reduce the CPU consumption when doing (de)compression. If using
+-these faster (de)compression method, less (de)compression threads
++Comparing to Zlib, Some faster (de)compression method such as LZ4
++and Quicklz can help to reduce the CPU consumption when doing (de)compression.
++If using these faster (de)compression method, less (de)compression threads
+ are needed when doing the migration.
+-- 
+2.27.0
+