Implement io_uring zero-copy send support

docs/config/network.en.md

@@ -34,6 +34,7 @@ between clients, OSDs and etcd.
 - [etcd_ws_keepalive_interval](#etcd_ws_keepalive_interval)
 - [etcd_min_reload_interval](#etcd_min_reload_interval)
 - [tcp_header_buffer_size](#tcp_header_buffer_size)
+- [min_zerocopy_send_size](#min_zerocopy_send_size)
 - [use_sync_send_recv](#use_sync_send_recv)
 
 ## osd_network
@@ -313,6 +314,34 @@ is received without an additional copy. You can try to play with this
 parameter and see how it affects random iops and linear bandwidth if you
 want.
 
+## min_zerocopy_send_size
+
+- Type: integer
+- Default: 32768
+
+OSDs and clients will attempt to use io_uring-based zero-copy TCP send
+for buffers larger than this number of bytes. Zero-copy send with io_uring is
+supported since Linux kernel version 6.1. Support is auto-detected and disabled
+automatically when not available. It can also be disabled explicitly by setting
+this parameter to a negative value.
+
+⚠️ Warning! Zero-copy send performance may vary greatly from CPU to CPU and from
+one kernel version to another. Generally, it tends to only make benefit with larger
+messages. With smaller messages (say, 4 KB), it may actually be slower. 32 KB is
+enough for almost all CPUs, but even smaller values are optimal for some of them.
+For example, 4 KB is OK for EPYC Milan/Genoa and 12 KB is OK for Xeon Ice Lake
+(but verify it yourself please).
+
+Verification instructions:
+1. Add `iommu=pt` into your Linux kernel command line and reboot.
+2. Upgrade your kernel. For example, it's very important to use 6.11+ with recent AMD EPYCs.
+3. Run some tests with the [send-zerocopy liburing example](https://github.com/axboe/liburing/blob/master/examples/send-zerocopy.c)
+   to find the minimal message size for which zero-copy is optimal.
+   Use `./send-zerocopy tcp -4 -R` at the server side and
+   `time ./send-zerocopy tcp -4 -b 0 -s BUFFER_SIZE -D SERVER_IP` at the client side with
+   `-z 0` (no zero-copy) and `-z 1` (zero-copy), and compare MB/s and used CPU time
+   (user+system).
+
 ## use_sync_send_recv
 
 - Type: boolean