[Qemu-devel] [PATCH 3/3] migration: optimization for one decompression thread

[Liang Li <liang.z.li@intel.com>]

When decompression thread count is set to 1, the current implementation
is inefficient because of the following reason:
1. Thread syncronization cost;
2. Data copy;

This patch optimizes the performance for the case of 1 decompress thread.
In this case, the compression is done in process_incoming_migration_co,
for some fast decompression algorithm, it can help to improve the
performance.

Signed-off-by: Liang Li <liang.z.li@intel.com>
---
 migration/ram.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index 0cc4f81..fc91997 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1414,6 +1414,9 @@ void migrate_decompress_threads_create(void)
     int i, thread_count;
 
     thread_count = migrate_decompress_threads();
+    if (thread_count == 1) {
+        return;
+    }
     decompress_threads = g_new0(QemuThread, thread_count);
     decomp_param = g_new0(DecompressParam, thread_count);
     compressed_data_buf = g_malloc0(compressBound(TARGET_PAGE_SIZE));
@@ -1432,8 +1435,11 @@ void migrate_decompress_threads_join(void)
 {
     int i, thread_count;
 
-    quit_decomp_thread = true;
     thread_count = migrate_decompress_threads();
+    if (thread_count == 1) {
+        return;
+    }
+    quit_decomp_thread = true;
     for (i = 0; i < thread_count; i++) {
         qemu_mutex_lock(&decomp_param[i].mutex);
         qemu_cond_signal(&decomp_param[i].cond);
@@ -1575,7 +1581,14 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
                 break;
             }
             qemu_get_buffer(f, compressed_data_buf, len);
-            decompress_data_with_multi_threads(compressed_data_buf, host, len);
+            if (migrate_decompress_threads() == 1) {
+                unsigned long pagesize = TARGET_PAGE_SIZE;
+                uncompress((Bytef *)host, &pagesize,
+                           (const Bytef *)compressed_data_buf, len);
+            } else {
+                decompress_data_with_multi_threads(compressed_data_buf,
+                                                   host, len);
+            }
             break;
         case RAM_SAVE_FLAG_XBZRLE:
             host = host_from_stream_offset(f, addr, flags);
-- 
1.9.1