From: Stephen Hemminger <stephen@networkplumber.org>
To: "Morten Brørup" <mb@smartsharesystems.com>
Cc: <dev@dpdk.org>, "Tyler Retzlaff" <roretzla@linux.microsoft.com>,
"Reshma Pattan" <reshma.pattan@intel.com>
Subject: Re: [PATCH v4 3/6] latencystats: do not use floating point
Date: Fri, 19 Apr 2024 15:45:16 -0700 [thread overview]
Message-ID: <20240419154516.6c934212@hermes.local> (raw)
In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35E9F3CE@smartserver.smartshare.dk>
On Fri, 19 Apr 2024 20:49:56 +0200
Morten Brørup <mb@smartsharesystems.com> wrote:
> > - /*
> > - * The average latency is measured using exponential moving
> > - * average, i.e. using EWMA
> > - * https://en.wikipedia.org/wiki/Moving_average
> > - */
> > - glob_stats->avg_latency +=
> > - alpha * (latency - glob_stats->avg_latency);
> > + glob_stats->avg_latency = latency;
> > + glob_stats->jitter = latency / 2;
>
> Setting jitter at first sample as latency / 2 is wrong.
> Jitter should remain zero at first sample.
Chose that because it is what the TCP RFC does.
RFC 6298
(2.2) When the first RTT measurement R is made, the host MUST set
SRTT <- R
RTTVAR <- R/2
RTO <- SRTT + max (G, K*RTTVAR)
The problem is that the smoothing constant in this code is quite small.
Also, the TCP RFC has, not sure if matters.
(2.3) When a subsequent RTT measurement R' is made, a host MUST set
RTTVAR <- (1 - beta) * RTTVAR + beta * |SRTT - R'|
SRTT <- (1 - alpha) * SRTT + alpha * R'
The value of SRTT used in the update to RTTVAR is its value
before updating SRTT itself using the second assignment. That
is, updating RTTVAR and SRTT MUST be computed in the above
order.
The above SHOULD be computed using alpha=1/8 and beta=1/4 (as
suggested in [JK88]).
After the computation, a host MUST update
RTO <- SRTT + max (G, K*RTTVAR)
next prev parent reply other threads:[~2024-04-19 22:45 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-08 19:50 [RFC] latencystats: performance overhaul Stephen Hemminger
2024-04-08 21:26 ` [PATCH v2] " Stephen Hemminger
2024-04-19 17:28 ` [PATCH v4 0/6] latencystats: cleanup Stephen Hemminger
2024-04-19 17:28 ` [PATCH v4 1/6] latencystats: replace use of VLA Stephen Hemminger
2024-04-19 17:28 ` [PATCH v4 2/6] latencystats: handle fractional cycles per ns Stephen Hemminger
2024-04-19 17:28 ` [PATCH v4 3/6] latencystats: do not use floating point Stephen Hemminger
2024-04-19 18:49 ` Morten Brørup
2024-04-19 22:45 ` Stephen Hemminger [this message]
2024-04-20 7:31 ` Morten Brørup
2024-04-19 17:28 ` [PATCH v4 4/6] latencystats: fix log messages Stephen Hemminger
2024-04-19 17:28 ` [PATCH v4 5/6] latencystats: update include files Stephen Hemminger
2024-04-19 17:28 ` [PATCH v4 6/6] latencystats: fix for pedantic warnings Stephen Hemminger
2024-04-22 15:21 ` [PATCH v5 0/9] latencystats: improve algorithms and tests Stephen Hemminger
2024-04-22 15:21 ` [PATCH v5 1/9] latencystats: replace use of VLA Stephen Hemminger
2024-04-22 15:21 ` [PATCH v5 2/9] latencystats: handle fractional cycles per ns Stephen Hemminger
2024-04-22 15:21 ` [PATCH v5 3/9] latencystats: do not use floating point Stephen Hemminger
2024-04-22 15:21 ` [PATCH v5 4/9] latencystats: fix log messages Stephen Hemminger
2024-04-22 15:21 ` [PATCH v5 5/9] latencystats: update include files Stephen Hemminger
2024-04-22 15:21 ` [PATCH v5 6/9] latencystats: enforce that unused callback function is NULL Stephen Hemminger
2024-04-22 15:21 ` [PATCH v5 7/9] latencystats: add metric for number of samples Stephen Hemminger
2024-04-22 15:21 ` [PATCH v5 8/9] test: use initialization in latencystats test Stephen Hemminger
2024-04-22 15:21 ` [PATCH v5 9/9] test: add more latencystats tests Stephen Hemminger
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