[PATCH] [TCP] Sysctl: document tcp_max_ssthresh (Limited Slow-Start)

[PATCH] [TCP] Sysctl: document tcp_max_ssthresh (Limited Slow-Start)

[Ilpo Järvinen]

[-- Attachment #1: Type: TEXT/PLAIN, Size: 1414 bytes --]


Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
---
 Documentation/networking/ip-sysctl.txt |   13 +++++++++++++
 1 files changed, 13 insertions(+), 0 deletions(-)

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index ce16e6a..44ba8d4 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -239,6 +239,19 @@ tcp_max_orphans - INTEGER
 	more aggressively. Let me to remind again: each orphan eats
 	up to ~64K of unswappable memory.
 
+tcp_max_ssthresh - INTEGER
+	Limited Slow-Start for TCP with Large Congestion Windows defined in
+	RFC3742. Limited slow-start is a mechanism to limit grow of the
+	congestion window on the region where congestion window is larger than
+	tcp_max_ssthresh. A TCP connection with a large congestion window could
+	have its congestion window increased by thousand (or even more)
+	segments per RTT by the traditional slow-start procedure which might be
+	counter-productive to TCP performance when packet losses start to
+	occur. With limited slow-start TCP increments congestion window at
+	most tcp_max_ssthresh/2 segments per RTT when the congestion window is
+	above tcp_max_ssthresh.
+	Default: 0 (off)
+
 tcp_max_syn_backlog - INTEGER
 	Maximal number of remembered connection requests, which are
 	still did not receive an acknowledgment from connecting client.
-- 
1.5.0.6

Re: [PATCH] [TCP] Sysctl: document tcp_max_ssthresh (Limited Slow-Start)

[Baruch Even]

Ilpo Järvinen wrote:
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
> ---
>  Documentation/networking/ip-sysctl.txt |   13 +++++++++++++
>  1 files changed, 13 insertions(+), 0 deletions(-)
> 
> diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
> index ce16e6a..44ba8d4 100644
> --- a/Documentation/networking/ip-sysctl.txt
> +++ b/Documentation/networking/ip-sysctl.txt
> @@ -239,6 +239,19 @@ tcp_max_orphans - INTEGER
>  	more aggressively. Let me to remind again: each orphan eats
>  	up to ~64K of unswappable memory.
>  
> +tcp_max_ssthresh - INTEGER
> +	Limited Slow-Start for TCP with Large Congestion Windows defined in
> +	RFC3742. Limited slow-start is a mechanism to limit grow of the

s/grow/growth/

> +	congestion window on the region where congestion window is larger than
> +	tcp_max_ssthresh. A TCP connection with a large congestion window could
> +	have its congestion window increased by thousand (or even more)
> +	segments per RTT by the traditional slow-start procedure which might be
> +	counter-productive to TCP performance when packet losses start to
> +	occur. With limited slow-start TCP increments congestion window at
> +	most tcp_max_ssthresh/2 segments per RTT when the congestion window is

I'm not a native English speaker but "at most" sounds a bit awkward to
me, maybe change it to "by no more than". But I'm sure someone can find
a better phrasing.

Baruch

Re: [PATCH] [TCP] Sysctl: document tcp_max_ssthresh (Limited Slow-Start)

[Rick Jones]

Baruch Even wrote:
> Ilpo Järvinen wrote:
> 
>>Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
>>---
>> Documentation/networking/ip-sysctl.txt |   13 +++++++++++++
>> 1 files changed, 13 insertions(+), 0 deletions(-)
>>
>>diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
>>index ce16e6a..44ba8d4 100644
>>--- a/Documentation/networking/ip-sysctl.txt
>>+++ b/Documentation/networking/ip-sysctl.txt
>>@@ -239,6 +239,19 @@ tcp_max_orphans - INTEGER
>> 	more aggressively. Let me to remind again: each orphan eats
>> 	up to ~64K of unswappable memory.
>> 
>>+tcp_max_ssthresh - INTEGER
>>+	Limited Slow-Start for TCP with Large Congestion Windows defined in
>>+	RFC3742. Limited slow-start is a mechanism to limit grow of the
> 
> 
> s/grow/growth/
> 
> 
>>+	congestion window on the region where congestion window is larger than
>>+	tcp_max_ssthresh. A TCP connection with a large congestion window could
>>+	have its congestion window increased by thousand (or even more)
>>+	segments per RTT by the traditional slow-start procedure which might be
>>+	counter-productive to TCP performance when packet losses start to
>>+	occur. With limited slow-start TCP increments congestion window at
>>+	most tcp_max_ssthresh/2 segments per RTT when the congestion window is
> 
> 
> I'm not a native English speaker but "at most" sounds a bit awkward to
> me, maybe change it to "by no more than". But I'm sure someone can find
> a better phrasing.

It could be "by at most" or "by no more than" but indeed it should not just be 
"at most."

Also, it should be "With limited slow-start TCP increments the congestion 
window..."   Combining with other nits yeilds:

Limited Slow-Start for TCP with Large Congestion Windows is defined in RFC3742. 
  Limited slow-start is a mechanism to limit growth of the congestion window 
when the congestion window is larger than tcp_max_ssthresh.  A TCP connection 
with a large congestion window could have its congestion window increased by 
thousands of segments (or more) per RTT via the traditional slow-start 
procedures. This might degrade TCP performance when packet losses start to 
occur.  With limited slow-start, TCP increments the congestion window by at most 
tcp_max_ssthresh/2 segments per RTT when the congestion window is is above 
tcp_max_ssthresh.

rick jones

as an asside, "tcp_max_ssthresh" sounds like the maximum value ssthresh can 
take-on.  is that correct, or is this more of a "once ssthresh is above this, 
behave in this new way?"  If that is the case, while the horse has probably left 
the barn, perhaps another name would be better?

> 
> Baruch
> -
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Re: [PATCH] [TCP] Sysctl: document tcp_max_ssthresh (Limited Slow-Start)

[John Heffner]

Rick Jones wrote:
> as an asside, "tcp_max_ssthresh" sounds like the maximum value ssthresh 
> can take-on.  is that correct, or is this more of a "once ssthresh is 
> above this, behave in this new way?"  If that is the case, while the 

I don't like it either, but you'll have to talk to Sally Floyd about 
that one.. ;)

In general, I would like the documentation to emphasize more how to set 
the parameter than describe the algorithm.  The max_ssthresh parameter 
should ideally be set to the bottleneck queue size, or more 
realistically a conservative value that's likely to be smaller than the 
bottleneck queue size.  When max_ssthresh is smaller than the bottleneck 
queue, (limited) slow start will not overflow it until cwnd has fully 
ramped up to the appropriate size.

   -John

Re: [PATCH] [TCP] Sysctl: document tcp_max_ssthresh (Limited Slow-Start)

[Ilpo Järvinen]

[-- Attachment #1: Type: TEXT/PLAIN, Size: 1932 bytes --]

On Fri, 18 May 2007, Baruch Even wrote:

> I'm not a native English speaker but "at most" sounds a bit awkward to
> me, maybe change it to "by no more than". But I'm sure someone can find
> a better phrasing.

Neither am I, but since nothing else seems to have came up, here is one 
with your proposal... In addition to your changes, I fixed the 
capitalization of the first sentence (which was a result of copy-paste 
from RFC's title).

[PATCH] [TCP] Sysctl: Document tcp_max_ssthresh (Limited Slow-Start)

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
---
 Documentation/networking/ip-sysctl.txt |   13 +++++++++++++
 1 files changed, 13 insertions(+), 0 deletions(-)

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index ce16e6a..b4d0a66 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -239,6 +239,19 @@ tcp_max_orphans - INTEGER
 	more aggressively. Let me to remind again: each orphan eats
 	up to ~64K of unswappable memory.
 
+tcp_max_ssthresh - INTEGER
+	Limited Slow-Start for TCP with large congestion windows defined in
+	RFC3742. Limited slow-start is a mechanism to limit growth of the
+	congestion window on the region where congestion window is larger than
+	tcp_max_ssthresh. A TCP connection with a large congestion window could
+	have its congestion window increased by thousand (or even more)
+	segments per RTT by the traditional slow-start procedure which might be
+	counter-productive to TCP performance when packet losses start to
+	occur. With limited slow-start TCP increments congestion window by no
+	more than tcp_max_ssthresh/2 segments per RTT when the congestion
+	window is above tcp_max_ssthresh.
+	Default: 0 (off)
+
 tcp_max_syn_backlog - INTEGER
 	Maximal number of remembered connection requests, which are
 	still did not receive an acknowledgment from connecting client.
-- 
1.5.0.6

Re: [PATCH] [TCP] Sysctl: document tcp_max_ssthresh (Limited Slow-Start)

[Ilpo Järvinen]

On Fri, 18 May 2007, John Heffner wrote:

> Rick Jones wrote:
> > as an asside, "tcp_max_ssthresh" sounds like the maximum value ssthresh can
> > take-on.  is that correct, or is this more of a "once ssthresh is above
> > this, behave in this new way?"  If that is the case, while the 
> 
> I don't like it either, but you'll have to talk to Sally Floyd about that
> one.. ;)
> 
> In general, I would like the documentation to emphasize more how to set the
> parameter than describe the algorithm.  The max_ssthresh parameter should
> ideally be set to the bottleneck queue size, or more realistically a
> conservative value that's likely to be smaller than the bottleneck queue
> size.  When max_ssthresh is smaller than the bottleneck queue, (limited) slow
> start will not overflow it until cwnd has fully ramped up to the appropriate
> size.

...You probably know such things much better than I do, so could you 
please write it then...

-- 
 i.

Re: [PATCH] [TCP] Sysctl: document tcp_max_ssthresh (Limited Slow-Start)

[Ilpo Järvinen]

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On Sat, 19 May 2007, Ilpo Järvinen wrote:

> but since nothing else seems to have came up

...Doh, I must have been gazing somewhere else while scanning the list 
mails, please ignore this, I apologize for the noise...

-- 
 i.

Re: [PATCH] [TCP] Sysctl: document tcp_max_ssthresh (Limited Slow-Start)

[Ilpo Järvinen]

On Fri, 18 May 2007, Rick Jones wrote:

> or is this more of a "once ssthresh is above this, behave in this new 
> way?"  

Not even that is very exact, since both ssthresh and cwnd have to be above 
max_ssthresh to get any new behavior...


-- 
 i.