In testing my recent Pull Request #775 for Issue #700, I came across some weird slowness compared to the old v0.23 jar we had been using for years. Pulling my hair out thinking it was something I had done, I decided to directly compare the official 0.32.0 release and the 0.32.1-SNAPSHOT (as it is today). Here are my sample code and findings:

 public void execute(String host, int port, String user, String pass, File file) throws Throwable { try (SSHClient ssh = new SSHClient()) { ssh.addHostKeyVerifier(new PromiscuousVerifier()); ssh.connect(host, port); ssh.authPassword(user, pass); try (SFTPClient sftp = ssh.newSFTPClient()) { final long startTime = System.nanoTime(); sftp.get(file.getName(), file.getName()); final double timeSeconds = TimeUnit.NANOSECONDS.toMillis (System.nanoTime() - startTime) / 1000.0; final double sizeKiB = file.length() / 1024.0; log.info(String.format("%1$,.1f KiB transferred in %2$,.1f seconds (%3$,.2f KiB/s)", sizeKiB, timeSeconds, (sizeKiB / timeSeconds))); } } } 

I have logback set to info for this, hence the copied log.info() from StringCopier.copy(). Here are the results:

sshj-0.32.0.jar:
18:41:04 [main] INFO n.s.sshj.transport.random.JCERandom - Creating new SecureRandom.
18:41:04 [main] INFO n.s.sshj.transport.TransportImpl - Client identity string: SSH-2.0-SSHJ_0.32.0
18:41:04 [main] INFO n.s.sshj.transport.TransportImpl - Server identity string: SSH-2.0-OpenSSH_7.2p2 Ubuntu-4ubuntu2.10
18:41:05 [main] INFO n.s.s.c.c.direct.SessionChannel - Will request sftp subsystem
18:49:05 [main] WARN net.schmizz.sshj.xfer.FileSystemFile - Could not set permissions for <file> to 1b0
18:49:05 [main] INFO sshj.resume.test2.SlownessTest - 11,161,746.5 KiB transferred in 480.8 seconds (23,215.57 KiB/s)
18:49:05 [main] INFO n.s.sshj.transport.TransportImpl - Disconnected - BY_APPLICATION

sshj-0.32.1-SNAPSHOT.jar:
18:49:42 [main] INFO n.s.sshj.transport.random.JCERandom - Creating new SecureRandom.
18:49:42 [main] INFO n.s.sshj.transport.TransportImpl - Client identity string: SSH-2.0-SSHJ_0.32.1_SNAPSHOT
18:49:42 [main] INFO n.s.sshj.transport.TransportImpl - Server identity string: SSH-2.0-OpenSSH_7.2p2 Ubuntu-4ubuntu2.10
18:49:43 [main] INFO n.s.s.c.c.direct.SessionChannel - Will request sftp subsystem
21:03:35 [main] WARN net.schmizz.sshj.xfer.FileSystemFile - Could not set permissions for <file> to 1b0
21:03:35 [main] INFO sshj.resume.test2.SlownessTest - 11,161,746.5 KiB transferred in 8,032.3 seconds (1,389.61 KiB/s)
21:03:35 [main] INFO n.s.sshj.transport.TransportImpl - Disconnected - BY_APPLICATION

This was done without compression as well. As you can see, 0.32.1-SNAPSHOT is slower by a factor of 16.7x. I am not familiar enough with the code base or the technology behind it to have a valid opinion on what is going on, but I figured it was worth pointing out to someone who might.

I have since replicated my #775 changes on a branch from the 0.32.0 tag and validated that it works as expected. The only difference between my two almost-identical branches is that against 0.32.0 I didn't need to modify RemoteFile.ReadAheadRemoteFileInputStream to add a (int, long) constructor which, under 0.32.1-SNAPSHOT, merely passed-through to the (int, long, long) constructor because in 0.32.0 it was already there. (apologies for the very confusing sentence, but I'm trying to be detailed here as I don't 100% know what I'm talking about).

It might be worth looking at recent changes to RemoteFile.ReadAheadRemoteFileInputStream, but that's just a complete guess merely based on my two branches.

You can see these at my fork https://github.com/brenttyler/sshj on the issue-700 branch (#775) and the 0.32.0-issue-700 branches (no Pull Request).