So this isn't an issue as much as a weird situation I am not fully
understanding. That said, if I can understand it then it might be a
benefit.
In the function client_process_net_input in clientloop.c if I increase
the size of buf[SSH_IOBUFSZ] to 128k I'm seeing a pretty substantial
performance improvement - mostly when using aes256-ctr.
For example; with the command
./ssh HostB -caes256-ctr "cat /dev/zero" > /dev/null
I'm seeing throughput of around 610MB/s on a 10Gb network.
When I use an unmodified version I'll see 480MB/s.
Same hosts, same command. The only difference being the size of buf in
client_process_net_input.
HostA is a Xeon x5675 @3Ghz. HostB is an AMD Ryzen 7 5800X.
My initial assumption is since HostA is CPU bound reducing the number of
reads has a significant impact. That said, a nearly 30% improvement
seems excessive for that to be all that's going on. Additionally, I'm
not seeing as much improvement using chachapoly. In that case, I'm only
seeing about a 20% improvement. 310MB/s for stock and 375MB/s for the
big buffer.
Additionally, I'm only seeing the improvement when HostB is sending the
data and HostA receiving. When HostA (the Xeon) is sending (cat
/dev/zero | ./ssh HostB "cat > /dev/null") then I'm seeing about 290MB/s
with either version.
I'm not suggesting any changes to the code. I'm just trying to
understand what might be happening as it could be the buffer size,
something with the CPU architecture, the switch I'm using, the distro
(HostA is fedora, HostB is ubuntu), etc. Any clues would be appreciated.
Here is the specific change I made:
diff --git a/clientloop.c b/clientloop.c
index bfcd50c2..8eebf9c2 100644
--- a/clientloop.c
+++ b/clientloop.c
@@ -600,7 +600,7 @@ client_suspend_self(struct sshbuf *bin, struct
sshbuf *bout, struct sshbuf *berr
static void
client_process_net_input(struct ssh *ssh, fd_set *readset)
{
- char buf[SSH_IOBUFSZ];
+ char buf[128*1024];
int r, len;
/*
Thanks,
Chris
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