[Qemu-devel] [PATCHv4 7/9] migration: do not sent zero pages in bulk stage

[Peter Lieven <pl@kamp.de>]

during bulk stage of ram migration if a page is a
zero page do not send it at all.
the memory at the destination reads as zero anyway.

even if there is an madvise with QEMU_MADV_DONTNEED
at the target upon receipt of a zero page I have observed
that the target starts swapping if the memory is overcommitted.
it seems that the pages are dropped asynchronously.

Signed-off-by: Peter Lieven <pl@kamp.de>
---
 arch_init.c                   |   24 ++++++++++++++++++++----
 hmp.c                         |    2 ++
 include/migration/migration.h |    2 ++
 migration.c                   |    3 ++-
 qapi-schema.json              |    6 ++++--
 qmp-commands.hx               |    3 ++-
 6 files changed, 32 insertions(+), 8 deletions(-)

diff --git a/arch_init.c b/arch_init.c
index 4c4caf4..c34a4af 100644
--- a/arch_init.c
+++ b/arch_init.c
@@ -181,6 +181,7 @@ int64_t xbzrle_cache_resize(int64_t new_size)
 /* accounting for migration statistics */
 typedef struct AccountingInfo {
     uint64_t dup_pages;
+    uint64_t skipped_pages;
     uint64_t norm_pages;
     uint64_t iterations;
     uint64_t xbzrle_bytes;
@@ -206,6 +207,16 @@ uint64_t dup_mig_pages_transferred(void)
     return acct_info.dup_pages;
 }
 
+uint64_t skipped_mig_bytes_transferred(void)
+{
+    return acct_info.skipped_pages * TARGET_PAGE_SIZE;
+}
+
+uint64_t skipped_mig_pages_transferred(void)
+{
+    return acct_info.skipped_pages;
+}
+
 uint64_t norm_mig_bytes_transferred(void)
 {
     return acct_info.norm_pages * TARGET_PAGE_SIZE;
@@ -438,10 +449,15 @@ static int ram_save_block(QEMUFile *f, bool last_stage)
             bytes_sent = -1;
             if (is_zero_page(p)) {
                 acct_info.dup_pages++;
-                bytes_sent = save_block_hdr(f, block, offset, cont,
-                                            RAM_SAVE_FLAG_COMPRESS);
-                qemu_put_byte(f, 0);
-                bytes_sent++;
+                if (!ram_bulk_stage) {
+                    bytes_sent = save_block_hdr(f, block, offset, cont,
+                                                RAM_SAVE_FLAG_COMPRESS);
+                    qemu_put_byte(f, 0);
+                    bytes_sent++;
+                } else {
+                    acct_info.skipped_pages++;
+                    bytes_sent = 0;
+                }
             } else if (migrate_use_xbzrle()) {
                 current_addr = block->offset + offset;
                 bytes_sent = save_xbzrle_page(f, p, current_addr, block,
diff --git a/hmp.c b/hmp.c
index b0a861c..e3e833e 100644
--- a/hmp.c
+++ b/hmp.c
@@ -173,6 +173,8 @@ void hmp_info_migrate(Monitor *mon, const QDict *qdict)
                        info->ram->total >> 10);
         monitor_printf(mon, "duplicate: %" PRIu64 " pages\n",
                        info->ram->duplicate);
+        monitor_printf(mon, "skipped: %" PRIu64 " pages\n",
+                       info->ram->skipped);
         monitor_printf(mon, "normal: %" PRIu64 " pages\n",
                        info->ram->normal);
         monitor_printf(mon, "normal bytes: %" PRIu64 " kbytes\n",
diff --git a/include/migration/migration.h b/include/migration/migration.h
index bb617fd..e2acec6 100644
--- a/include/migration/migration.h
+++ b/include/migration/migration.h
@@ -96,6 +96,8 @@ extern SaveVMHandlers savevm_ram_handlers;
 
 uint64_t dup_mig_bytes_transferred(void);
 uint64_t dup_mig_pages_transferred(void);
+uint64_t skipped_mig_bytes_transferred(void);
+uint64_t skipped_mig_pages_transferred(void);
 uint64_t norm_mig_bytes_transferred(void);
 uint64_t norm_mig_pages_transferred(void);
 uint64_t xbzrle_mig_bytes_transferred(void);
diff --git a/migration.c b/migration.c
index 185d112..7fb2147 100644
--- a/migration.c
+++ b/migration.c
@@ -197,11 +197,11 @@ MigrationInfo *qmp_query_migrate(Error **errp)
         info->ram->remaining = ram_bytes_remaining();
         info->ram->total = ram_bytes_total();
         info->ram->duplicate = dup_mig_pages_transferred();
+        info->ram->skipped = skipped_mig_pages_transferred();
         info->ram->normal = norm_mig_pages_transferred();
         info->ram->normal_bytes = norm_mig_bytes_transferred();
         info->ram->dirty_pages_rate = s->dirty_pages_rate;
 
-
         if (blk_mig_active()) {
             info->has_disk = true;
             info->disk = g_malloc0(sizeof(*info->disk));
@@ -227,6 +227,7 @@ MigrationInfo *qmp_query_migrate(Error **errp)
         info->ram->remaining = 0;
         info->ram->total = ram_bytes_total();
         info->ram->duplicate = dup_mig_pages_transferred();
+        info->ram->skipped = skipped_mig_pages_transferred();
         info->ram->normal = norm_mig_pages_transferred();
         info->ram->normal_bytes = norm_mig_bytes_transferred();
         break;
diff --git a/qapi-schema.json b/qapi-schema.json
index fdaa9da..b737460 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -496,7 +496,9 @@
 #
 # @total: total amount of bytes involved in the migration process
 #
-# @duplicate: number of duplicate pages (since 1.2)
+# @duplicate: number of duplicate (zero) pages (since 1.2)
+#
+# @skipped: number of skipped zero pages (since 1.5)
 #
 # @normal : number of normal pages (since 1.2)
 #
@@ -510,7 +512,7 @@
 { 'type': 'MigrationStats',
   'data': {'transferred': 'int', 'remaining': 'int', 'total': 'int' ,
            'duplicate': 'int', 'normal': 'int', 'normal-bytes': 'int',
-           'dirty-pages-rate' : 'int' } }
+           'dirty-pages-rate' : 'int', 'skipped': 'int' } }
 
 ##
 # @XBZRLECacheStats
diff --git a/qmp-commands.hx b/qmp-commands.hx
index b370060..fed74c6 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -2442,7 +2442,8 @@ The main json-object contains the following:
          - "transferred": amount transferred (json-int)
          - "remaining": amount remaining (json-int)
          - "total": total (json-int)
-         - "duplicate": number of duplicated pages (json-int)
+         - "duplicate": number of duplicated (zero) pages (json-int)
+         - "skipped": number of skipped zero pages (json-int)
          - "normal" : number of normal pages transferred (json-int)
          - "normal-bytes" : number of normal bytes transferred (json-int)
 - "disk": only present if "status" is "active" and it is a block migration,
-- 
1.7.9.5