On Thu, 6 May 2010, Xu, Qiang (FXSGSC) wrote:

> By the way, from the comment above defining CURL_MAX_WRITE_SIZE to 16384, it
> seems you have met some similar problems with Windows SFTP server, esp. when
> it comes to uploading. Could you tell me the full story behind it? It seems
> to me that the original buffer size limit is 20KB, and it is changed to 16KB
> later.

You need to remember that libcurl is multi-protocol. We set the buffer size to
16K *years* before libcurl could even do SFTP.

The reason is simply that back in the days we found out that the Windows TCP
stack simply behaved very differently depending on what buffer sizes we used,
and for some reason 16K seemed to be a pretty good size to get decent enough
throughput at the time we tested it. We got worse performance by increasing
the buffer size. We also don't want to have the buffer size too big since each
handle will have this buffer size allocated (and at times even two of these
buffers).

Tests done in modern times have also verified that 16K is still a pretty
decent size for most operating systems.

We've designed the header to easily allow builders to customize the size in
the cases you actually find a size that works better for a particular
use-case. We have however not found any better size than 16K that works good
enough genericly "all over".

-- 
  / daniel.haxx.se
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