[libvirt PATCH 1/2] remote: make ssh-helper massively faster

[Michal Prívozník]

On 11/25/20 7:04 PM, Daniel P. Berrangé wrote:
> It was reported that the performance of tunnelled migration and
> volume upload/download regressed in 6.9.0, when the virt-ssh-helper
> is used for remote SSH tunnelling instead of netcat.
> 
> When seeing data available to read from stdin, or the socket,
> the current code will allocate at most 1k of extra space in
> the buffer it has.
> 
> After writing data to the socket, or stdout, if more than 1k
> of extra space is in the buffer, it will reallocate to free
> up that space.
> 
> This results in a huge number of mallocs when doing I/O, as
> well as a huge number of syscalls since at most 1k of data
> will be read/written at a time.
> 
> Also if writing blocks for some reason, it will continue to
> read data with no memory bound which is bad.
> 
> This changes the code to use a 1 MB fixed size buffer in each
> direction. If that buffer becomes full, it will update the
> watches to stop reading more data. It will never reallocate
> the buffer at runtime.
> 
> This increases the performance by orders of magnitude.
> 
> Signed-off-by: Daniel P. Berrangé <berrange at redhat.com>
> ---
>   src/remote/remote_ssh_helper.c | 113 ++++++++++++++++++++-------------
>   1 file changed, 68 insertions(+), 45 deletions(-)
> 
> diff --git a/src/remote/remote_ssh_helper.c b/src/remote/remote_ssh_helper.c
> index 0da55c1d1f..8ed7e64507 100644
> --- a/src/remote/remote_ssh_helper.c
> +++ b/src/remote/remote_ssh_helper.c
> @@ -32,6 +32,8 @@
>   
>   #define VIR_FROM_THIS VIR_FROM_REMOTE
>   
> +#define SSH_BUF_SIZE (1024 * 1024)


In theory, our RPC messages can be up to 32MB long (since 
v3.4.0-rc1~26). Do you see any improvements with bigger buffer? But 
since this buffer is allocated at all times it's a trade off I guess.

Michal