[Spice-devel] [PATCH] Use non-zero data for initial ping

[Dan McGee]

This prevents compression over things such as VPN links misleading us on
available bandwidth. The page used before was 4K worth of zeroes, which
isn't a very realistic test at all.

We now use a generated sequence of bytes that vary in value and offset
between consecutive values, causing compression programs to have a much
harder time reducing our ping packets to nothing.

Here are some comparisons of before/after using a small standalone
program that generates bytes in the same fashion:

Before:
    4096 zerodata
    43   zerodata.bz2
    38   zerodata.gz
    92   zerodata.xz

After:
    4096 seqdata
    1213 seqdata.bz2
    4119 seqdata.gz
    3208 seqdata.xz

Signed-off-by: Dan McGee <dpmcgee at gmail.com>
---

This was a TODO item on the following page:
http://spice-space.org/page/Features/Bandwidth_monitoring

The standalone test program is below if anyone wants to try it out; simply pass
'4096' or similar as the first argument and you will get 4096 "random" bytes on
stdout that you can play with.

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
	int i, total;

	total = atoi(argv[1]);

	for(i = 0; i < total; i++) {
		div_t result = div(i, 256);
		if(result.quot % 2 == 0) {
			fputc((result.quot + 1) * result.rem, stdout);
		} else {
			fputc(512 - (result.quot * result.rem), stdout);
		}
	}
	return 0;
}


 server/main_channel.c |   24 ++++++++++++++++--------
 1 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/server/main_channel.c b/server/main_channel.c
index f7e1ab0..2ce20c0 100644
--- a/server/main_channel.c
+++ b/server/main_channel.c
@@ -22,6 +22,7 @@
 #include <inttypes.h>
 #include <stdint.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <unistd.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
@@ -45,8 +46,6 @@
 #include "red_channel.h"
 #include "generated_marshallers.h"
 
-#define ZERO_BUF_SIZE 4096
-
 #define REDS_MAX_SEND_IOVEC 100
 
 #define NET_TEST_WARMUP_BYTES 0
@@ -54,8 +53,6 @@
 
 #define PING_INTERVAL (1000 * 10)
 
-static uint8_t zero_page[ZERO_BUF_SIZE] = {0};
-
 typedef struct RedsOutItem RedsOutItem;
 struct RedsOutItem {
     PipeItem base;
@@ -335,17 +332,28 @@ static void main_channel_marshall_ping(SpiceMarshaller *m, int size, int ping_id
 {
     struct timespec time_space;
     SpiceMsgPing ping;
+    uint8_t *ping_data;
+    int i;
 
     ping.id = ping_id;
     clock_gettime(CLOCK_MONOTONIC, &time_space);
     ping.timestamp = time_space.tv_sec * 1000000LL + time_space.tv_nsec / 1000LL;
     spice_marshall_msg_ping(m, &ping);
 
-    while (size > 0) {
-        int now = MIN(ZERO_BUF_SIZE, size);
-        size -= now;
-        spice_marshaller_add_ref(m, zero_page, now);
+    ping_data = spice_malloc(size);
+    /* this produces ascending and descending byte runs which vary in offset
+     * every 512 bytes, preventing prevents compression from being able to
+     * influence the resulting size of the ping data too much */
+    for(i = 0; i < size; i++) {
+        div_t result = div(i, 256);
+        if(result.quot % 2 == 0) {
+            ping_data[i] = (result.quot + 1) * result.rem;
+        } else {
+            ping_data[i] = 512 - (result.quot * result.rem);
+        }
     }
+
+    spice_marshaller_add(m, ping_data, size);
 }
 
 void main_channel_push_mouse_mode(MainChannel *main_chan, int current_mode,
-- 
1.7.9