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

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(a)redhat.com&gt; > --- > 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.