I'm experimenting with an application where adjusting the retransmit
and rate control policies could optimize performance. For my
application, latency, reliability and throughput are all concerns,
with latency being the largest concern, followed by reliability and
throughput.
I would appreciate comments on the changes I am considering for
rc_80211_minstrel.c:
-------------------
A) Increase EWMA level to favor newer data more heavily, as in this
application the link is in a fixed location (so past data is basically
flat) but may experience occasional, random interference (generally
very brief) and we want the best possible retry bitrates.
Change line 536:
mp->ewma_level = 75;
To:
mp->ewma_level = 90;
B) Increase EWMA update interval. The goal is to make the system jump
to a lower rate much more quickly if needed, to reduce the risk of
packet loss or very large retransmit delays.
Change line 551:
mp->update_interval = 100;
to:
mp->update_interval = 10;
Question: Is an update interval this tight going to impose a CPU
performance issue? In these situations, we have plenty of CPU
available (nor are we concerned about power usage), so I am skeptical
this will be a major problem, but could this lead to a slowdown in the
throughput due to internal bottlenecks / lack of threading?
C) Reduce the maximum segment size to reduce air latency and reduce
the amount of data to be re-transmitted in the event of very brief
packet losses. 1 ms at 300 mbit (MCS15) with short preambles only
drops maximum frame size from ~64K to ~16K, which should not be too
huge a performance impact, though it may be noticeable for some loads.
I can do this fairly safely as I know I will normally be operating at
a very high bitrate, whereas if I was at a very low bitrate normally,
the throughput impact could be much larger.
Change line 539:
mp->segment_size = 6000;
to:
mp->segment_size = 500;
Question: Have I changed all the appropriate code, or does code
elsewhere need changing to alter the maximum segment size for all
traffic? Is reducing the maximum segment size going to scale the
maximum segment size down smoothly, without breaking anything else?
D) Standardize the maximum number of retries
Change line 541:
if (hw->max_rate_tries > 0)
mp->max_retry = hw->max_rate_tries;
else
/* safe default, does not necessarily have to match hw properties */
mp->max_retry = 7;
to:
/* if (hw->max_rate_tries > 0)
mp->max_retry = hw->max_rate_tries;
else
/* safe default, does not necessarily have to match hw properties */
mp->max_retry = 7;
Question: Will this work properly with all cards?
E) Reduce the lookaround rate, as our link is generally stable, and
unexpected events that occur are very abrupt and unlikely to be
detected in advance through the look-around. This will hopefully help
throughput a bit, without hurting reliability.
Change line 532:
mp->lookaround_rate = 5;
mp->lookaround_rate_mrr = 10;
to:
mp->lookaround_rate = 2;
mp->lookaround_rate_mrr = 4;
---------------
These changes seem like they'd help with my goals, but I'm not
experienced with this codebase, so I could be breaking a whole range
of things. Also, I am intending to use these changes with Atheros
cards in access point mode and Intel 5300 cards in station mode. Any
special considerations for those cards, especially
I'd appreciate comments, suggestion, discussion, etc. before I build
these changes and test in the field. (Field testing is a bit of a
project...)
-Galen
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