[Qemu-devel] [PULL 3/5] migration/block: limit the number of parallel I/O requests

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: Peter Lieven <pl@kamp.de>

the current implementation submits up to 512 I/O requests in parallel
which is much to high especially for a background task.
This patch adds a maximum limit of 16 I/O requests that can
be submitted in parallel to avoid monopolizing the I/O device.

Signed-off-by: Peter Lieven <pl@kamp.de>
Message-Id: <1520507908-16743-5-git-send-email-pl@kamp.de>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 migration/block.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/migration/block.c b/migration/block.c
index 5c03632257..03bbba61cb 100644
--- a/migration/block.c
+++ b/migration/block.c
@@ -37,6 +37,7 @@
 #define MAX_IS_ALLOCATED_SEARCH (65536 * BDRV_SECTOR_SIZE)
 
 #define MAX_IO_BUFFERS 512
+#define MAX_PARALLEL_IO 16
 
 //#define DEBUG_BLK_MIGRATION
 
@@ -775,6 +776,7 @@ static int block_save_iterate(QEMUFile *f, void *opaque)
     while ((block_mig_state.submitted +
             block_mig_state.read_done) * BLOCK_SIZE <
            qemu_file_get_rate_limit(f) &&
+           block_mig_state.submitted < MAX_PARALLEL_IO &&
            (block_mig_state.submitted + block_mig_state.read_done) <
            MAX_IO_BUFFERS) {
         blk_mig_unlock();
-- 
2.14.3