[Qemu-devel] [PATCH 06/20] Improve qemu-nbd performance by 4400 %

[Kevin Wolf]

From: Laurent Vivier <address@hidden>

This patch allows to reduce the boot time from an NBD server from 225 seconds to
5 seconds (time between the "boot cd:0" and the kernel init) for the
following command lines:

./qemu-nbd -t ../ISO/debian-500-powerpc-netinst.iso
and
./ppc-softmmu/qemu-system-ppc -cdrom nbd:localhost:1024

This patch combines the reply header and payload send operation.

Signed-off-by: Laurent Vivier <address@hidden>
Signed-off-by: Kevin Wolf <address@hidden>
---
 nbd.c |   25 ++++++++++++++++++-------
 1 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/nbd.c b/nbd.c
index 011b50f..4bf2eb7 100644
--- a/nbd.c
+++ b/nbd.c
@@ -49,6 +49,7 @@
 
 /* This is all part of the "official" NBD API */
 
+#define NBD_REPLY_SIZE         (4 + 4 + 8)
 #define NBD_REQUEST_MAGIC       0x25609513
 #define NBD_REPLY_MAGIC         0x67446698
 
@@ -588,7 +589,7 @@ static int nbd_receive_request(int csock, struct 
nbd_request *request)
 
 int nbd_receive_reply(int csock, struct nbd_reply *reply)
 {
-       uint8_t buf[4 + 4 + 8];
+       uint8_t buf[NBD_REPLY_SIZE];
        uint32_t magic;
 
        memset(buf, 0xAA, sizeof(buf));
@@ -655,9 +656,9 @@ int nbd_trip(BlockDriverState *bs, int csock, off_t size, 
uint64_t dev_offset,
        if (nbd_receive_request(csock, &request) == -1)
                return -1;
 
-       if (request.len > data_size) {
+       if (request.len + NBD_REPLY_SIZE > data_size) {
                LOG("len (%u) is larger than max len (%u)",
-                   request.len, data_size);
+                   request.len + NBD_REPLY_SIZE, data_size);
                errno = EINVAL;
                return -1;
        }
@@ -687,7 +688,8 @@ int nbd_trip(BlockDriverState *bs, int csock, off_t size, 
uint64_t dev_offset,
        case NBD_CMD_READ:
                TRACE("Request type is READ");
 
-               if (bdrv_read(bs, (request.from + dev_offset) / 512, data,
+               if (bdrv_read(bs, (request.from + dev_offset) / 512,
+                             data + NBD_REPLY_SIZE,
                              request.len / 512) == -1) {
                        LOG("reading from file failed");
                        errno = EINVAL;
@@ -697,12 +699,21 @@ int nbd_trip(BlockDriverState *bs, int csock, off_t size, 
uint64_t dev_offset,
 
                TRACE("Read %u byte(s)", request.len);
 
-               if (nbd_send_reply(csock, &reply) == -1)
-                       return -1;
+               /* Reply
+                  [ 0 ..  3]    magic   (NBD_REPLY_MAGIC)
+                  [ 4 ..  7]    error   (0 == no error)
+                  [ 7 .. 15]    handle
+                */
+
+               cpu_to_be32w((uint32_t*)data, NBD_REPLY_MAGIC);
+               cpu_to_be32w((uint32_t*)(data + 4), reply.error);
+               cpu_to_be64w((uint64_t*)(data + 8), reply.handle);
 
                TRACE("Sending data to client");
 
-               if (write_sync(csock, data, request.len) != request.len) {
+               if (write_sync(csock, data,
+                              request.len + NBD_REPLY_SIZE) !=
+                              request.len + NBD_REPLY_SIZE) {
                        LOG("writing to socket failed");
                        errno = EINVAL;
                        return -1;
-- 
1.7.2.2