speed caps: not based on average speeds anymore

Speed limits (from CURLOPT_MAX_RECV_SPEED_LARGE &
CURLOPT_MAX_SEND_SPEED_LARGE) were applied simply by comparing limits
with the cumulative average speed of the entire transfer; While this
might work at times with good/constant connections, in other cases it
can result to the limits simply being "ignored" for more than "short
bursts" (as told in man page).

Consider a download that goes on much slower than the limit for some
time (because bandwidth is used elsewhere, server is slow, whatever the
reason), then once things get better, curl would simply ignore the limit
up until the average speed (since the beginning of the transfer) reached
the limit.  This could prove the limit useless to effectively avoid
using the entire bandwidth (at least for quite some time).

So instead, we now use a "moving starting point" as reference, and every
time at least as much as the limit as been transferred, we can reset
this starting point to the current position. This gets a good limiting
effect that applies to the "current speed" with instant reactivity (in
case of sudden speed burst).

Closes #971

3 files changed, 5 insertions, 10 deletions

diff --git a/docs/curl.1 b/docs/curl.1
index f0ce3a791..7a6cbbd5a 100644
--- a/docs/curl.1
+++ b/docs/curl.1
@@ -1024,10 +1024,6 @@ The given speed is measured in bytes/second, unless a suffix is appended.
 Appending 'k' or 'K' will count the number as kilobytes, 'm' or M' makes it
 megabytes, while 'g' or 'G' makes it gigabytes. Examples: 200K, 3m and 1G.
 
-The given rate is the average speed counted during the entire transfer. It
-means that curl might use higher transfer speeds in short bursts, but over
-time it uses no more than the given rate.
-
 If you also use the \fI-Y, --speed-limit\fP option, that option will take
 precedence and might cripple the rate-limiting slightly, to help keeping the
 speed-limit logic working.
diff --git a/docs/libcurl/opts/CURLOPT_MAX_RECV_SPEED_LARGE.3 b/docs/libcurl/opts/CURLOPT_MAX_RECV_SPEED_LARGE.3
index 031f2cd39..c99ff61e3 100644
--- a/docs/libcurl/opts/CURLOPT_MAX_RECV_SPEED_LARGE.3
+++ b/docs/libcurl/opts/CURLOPT_MAX_RECV_SPEED_LARGE.3
@@ -31,9 +31,8 @@ CURLcode curl_easy_setopt(CURL *handle, CURLOPT_MAX_RECV_SPEED_LARGE,
                           curl_off_t speed);
 .SH DESCRIPTION
 Pass a curl_off_t as parameter.  If a download exceeds this \fIspeed\fP
-(counted in bytes per second) on cumulative average during the transfer, the
-transfer will pause to keep the average rate less than or equal to the
-parameter value. Defaults to unlimited speed.
+(counted in bytes per second) the transfer will pause to keep the speed less
+than or equal to the parameter value. Defaults to unlimited speed.
 
 This option doesn't affect transfer speeds done with FILE:// URLs.
 .SH DEFAULT
diff --git a/docs/libcurl/opts/CURLOPT_MAX_SEND_SPEED_LARGE.3 b/docs/libcurl/opts/CURLOPT_MAX_SEND_SPEED_LARGE.3
index c2c6336f1..7f3efe57c 100644
--- a/docs/libcurl/opts/CURLOPT_MAX_SEND_SPEED_LARGE.3
+++ b/docs/libcurl/opts/CURLOPT_MAX_SEND_SPEED_LARGE.3
@@ -31,9 +31,9 @@ CURLcode curl_easy_setopt(CURL *handle, CURLOPT_MAX_SEND_SPEED_LARGE,
                           curl_off_t maxspeed);
 .SH DESCRIPTION
 Pass a curl_off_t as parameter with the \fImaxspeed\fP.  If an upload exceeds
-this speed (counted in bytes per second) on cumulative average during the
-transfer, the transfer will pause to keep the average rate less than or equal
-to the parameter value.  Defaults to unlimited speed.
+this speed (counted in bytes per second) the transfer will pause to keep the
+speed less than or equal to the parameter value.  Defaults to unlimited
+speed.
 
 This option doesn't affect transfer speeds done with FILE:// URLs.
 .SH DEFAULT