WinScp SFTP upload is deadly slow

Recently we updated our WinScp from 5.1.3 to 5.7.3, the problem is our SFTP upload speed is very low compared to the older version (new: 250KB/s vs old: 2.7MB/s), but normal FTP is not affected, we create sessions with default parameters in both versions, unfortunately I couldn't found any documentation regarding to this problem on the official website.

it seems the problem is present from version 5.2.5 and above, however I noticed during my tests that it only occured with CoreFTP server for me, I tested it with Solarwind FTP server and everything worked smoothly, actually the newer versions were faster with the latter. then I tested with Psftp 0.64 client and CoreFTP but it didn't had the problem. I doubled checked for CPU-bound bug and visually my CPU usage is very low (on all cores).

I included 3 logs

1- 5.2.5 with CoreFTP (has the problem)
2- 5.2.4 with CoreFTP (OK)
3- 5.2.5 with Solarwind Server (OK)

Thanks for your report.
Can you update the "5.2.5-coreftp" log to include also the header?

Your superuser.com post:
https://superuser.com/q/924268/213663

martin wrote:

Thanks for your report.
Can you update the "5.2.5-coreftp" log to include also the header?

Your superuser.com post:
https://superuser.com/q/924268/213663

Sorry for the late response, I uploaded a Level 2 log for version 5.2.5 with core FTP as server.

Best Regards
Hessam

Since 5.2.5 WinSCP supports HMAC-SHA-256. It is computationally more demanding than older HMAC-SHA1.

As you claim not to see a high CPU usage on a client side, it might be due to an inefficient implementation on the server-side.
But that would not explain why you do not see any problem with psftp 0.64, which should use HMAC-SHA-256 too. Can you post also its log file?

You do not see the problem with Solarwind server as it does not support HMAC-SHA-256, so WinSCP needs to fallback to HMAC-SHA1.

martin wrote:

Since 5.2.5 WinSCP supports HMAC-SHA-256. It is computationally more demanding than older HMAC-SHA1.

As you claim not to see a high CPU usage on a client side, it might be due to an inefficient implementation on the server-side.
But that would not explain why you do not see any problem with psftp 0.64, which should use HMAC-SHA-256 too. Can you post also its log file?

You do not see the problem with Solarwind server as it does not support HMAC-SHA-256, so WinSCP needs to fallback to HMAC-SHA1.

here is the verbose output of psftp 0.64 to Core FTP server

Indeed the psftp uses the same MAC. So I do not see any other possibility than a local CPU issue. Are you sure that one of the cores is not maxed during the transfer with WinSCP?

martin wrote:

Indeed the psftp uses the same MAC. So I do not see any other possibility than a local CPU issue. Are you sure that one of the cores is not maxed during the transfer with WinSCP?

yes I'm pretty sure the problem should be something else, I attached 2 screenshots from my cpu usage during file transfer for both 5.2.5 and 5.2.4, actually my cpu usage is much higher in 5.2.4 (which is understandable because of higher file transfer speed)

System Specification:

AMD FX-6100, 4GB Ram
Windows 8.1 x64

How can I fallback (force) to HMAC-SHA1 as a temporary workaround in latest version of WinSCP?

I have sent you an email with a development version of WinSCP to address you have used to register on this forum. Please test it and let me know if it performs differently.

martin wrote:

I have sent you an email with a development version of WinSCP to address you have used to register on this forum. Please test it and let me know if it performs differently.

I tested with the development version you sent me and unfortunately I have to say there was no difference in performance compared to 5.2.5 an above. Do you want me to send you the logs from this version too?

Best Regards

hessamj wrote:

Do you want me to send you the logs from this version too?

Yes please (Debug 1 level)