[Bug 2927] scp -l can't limit the bandwidth at the first several seconds

bugzilla-daemon at bugzilla.mindrot.org bugzilla-daemon at bugzilla.mindrot.org
Fri Dec 7 15:02:02 AEDT 2018


Darren Tucker <dtucker at dtucker.net> changed:

           What    |Removed                     |Added
                 CC|                            |djm at mindrot.org,
                   |                            |dtucker at dtucker.net
   Attachment #3212|                            |ok?(djm at mindrot.org)
              Flags|                            |

--- Comment #2 from Darren Tucker <dtucker at dtucker.net> ---
Created attachment 3212
  --> https://bugzilla.mindrot.org/attachment.cgi?id=3212&action=edit
Fix scp bwlimit calculation.

Looking at the misc.c:bandwidth* functions I think there's two

> bandwidth_limit(struct bwlimit *bw, size_t read_len)
> { [...]
>        if (!timerisset(&bw->bwstart)) {
>                monotime_tv(&bw->bwstart);
>                return;
>        }
>        bw->lamt += read_len;

The first (minor) problem: the first buffer written (16k, by default)
is not accounted for at all because it returns before it gets to the
code that accounts for it.

>        if (bw->lamt < bw->thresh)
>                return;

This is the main problem: bw->thresh is set to the bandwidth limit (in
kbit/s), which means that for any non-trivial bandwidth limit it'll
receive many writes before it even considers inserting a delay, so you
get an initial burst followed by pause until the average drops back to
the intended rate.

This patch moves accounting so the first block is accounted for, and
sets the initial threshold such that it'll start computing the rate on
the second and subsequent blocks.

Without patch:
$ ./scp -l 1000 /tmp/1MB  localhost:/dev/null
1MB                    0%    0     0.0KB/s   --:-- ETA
1MB                  100% 1024KB   1.0MB/s   00:01    
1MB                  100% 1024KB 511.8KB/s   00:02    
1MB                  100% 1024KB 341.2KB/s   00:03    
1MB                  100% 1024KB 255.9KB/s   00:04    
1MB                  100% 1024KB 204.7KB/s   00:05    
1MB                  100% 1024KB 170.6KB/s   00:06    
1MB                  100% 1024KB 146.2KB/s   00:07    
1MB                  100% 1024KB 127.9KB/s   00:08 

With patch:
$ ./scp -l 1000 /tmp/1MB  localhost:/dev/null
1MB                    0%    0     0.0KB/s   --:-- ETA
1MB                   12%  128KB 127.9KB/s   00:07 ETA
1MB                   25%  256KB 127.9KB/s   00:06 ETA
1MB                   37%  384KB 127.9KB/s   00:05 ETA
1MB                   50%  512KB 127.9KB/s   00:04 ETA
1MB                   60%  624KB 126.3KB/s   00:03 ETA
1MB                   73%  752KB 126.5KB/s   00:02 ETA
1MB                   85%  880KB 126.6KB/s   00:01 ETA
1MB                   98% 1008KB 126.8KB/s   00:00 ETA
1MB                  100% 1024KB 124.6KB/s   00:08

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