diff options
author | Gerrit Renker <gerrit@erg.abdn.ac.uk> | 2008-09-04 01:30:19 -0400 |
---|---|---|
committer | Gerrit Renker <gerrit@erg.abdn.ac.uk> | 2008-09-04 01:45:33 -0400 |
commit | de6f2b59e5cd15a8772adb732a1d80e141a77115 (patch) | |
tree | 3ed9cb3498826675d4a56896fde7d2520f9aa122 /net/dccp/ccids/ccid3.h | |
parent | b2e317f4b5ae73733963c702fae0f246d234100b (diff) |
dccp ccid-3: Bug fix for the inter-packet scheduling algorithm
This fixes a subtle bug in the calculation of the inter-packet gap and shows
that t_delta, as it is currently used, is not needed. And hence replaced.
The algorithm from RFC 3448, 4.6 below continually computes a send time t_nom,
which is initialised with the current time t_now; t_gran = 1E6 / HZ specifies
the scheduling granularity, s the packet size, and X the sending rate:
t_distance = t_nom - t_now; // in microseconds
t_delta = min(t_ipi, t_gran) / 2; // `delta' parameter in microseconds
if (t_distance >= t_delta) {
reschedule after (t_distance / 1000) milliseconds;
} else {
t_ipi = s / X; // inter-packet interval in usec
t_nom += t_ipi; // compute the next send time
send packet now;
}
1) Description of the bug
-------------------------
Rescheduling requires a conversion into milliseconds, due to this call chain:
* ccid3_hc_tx_send_packet() returns a timeout in milliseconds,
* this value is converted by msecs_to_jiffies() in dccp_write_xmit(),
* and finally used as jiffy-expires-value for sk_reset_timer().
The highest jiffy resolution with HZ=1000 is 1 millisecond, so using a higher
granularity does not make much sense here.
As a consequence, values of t_distance < 1000 are truncated to 0. This issue
has so far been resolved by using instead
if (t_distance >= t_delta + 1000)
reschedule after (t_distance / 1000) milliseconds;
The bug is in artificially inflating t_delta to t_delta' = t_delta + 1000. This
is unnecessarily large, a more adequate value is t_delta' = max(t_delta, 1000).
2) Consequences of using the corrected t_delta'
-----------------------------------------------
Since t_delta <= t_gran/2 = 10^6/(2*HZ), we have t_delta <= 1000 as long as
HZ >= 500. This means that t_delta' = max(1000, t_delta) is constant at 1000.
On the other hand, when using a coarse HZ value of HZ < 500, we have three
sub-cases that can all be reduced to using another constant of t_gran/2.
(a) The first case arises when t_ipi > t_gran. Here t_delta' is the constant
t_delta' = max(1000, t_gran/2) = t_gran/2.
(b) If t_ipi <= 2000 < t_gran = 10^6/HZ usec, then t_delta = t_ipi/2 <= 1000,
so that t_delta' = max(1000, t_delta) = 1000 < t_gran/2.
(c) If 2000 < t_ipi <= t_gran, we have t_delta' = max(t_delta, 1000) = t_ipi/2.
In the second and third cases we have delay values less than t_gran/2, which is
in the order of less than or equal to half a jiffy.
How these are treated depends on how fractions of a jiffy are handled: they
are either always rounded down to 0, or always rounded up to 1 jiffy (assuming
non-zero values). In both cases the error is on average in the order of 50%.
Thus we are not increasing the error when in the second/third case we replace
a value less than t_gran/2 with 0, by setting t_delta' to the constant t_gran/2.
3) Summary
----------
Fixing (1) and considering (2), the patch replaces t_delta with a constant,
whose value depends on CONFIG_HZ, changing the above algorithm to:
if (t_distance >= t_delta')
reschedule after (t_distance / 1000) milliseconds;
where t_delta' = 10^6/(2*HZ) if HZ < 500, and t_delta' = 1000 otherwise.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Diffstat (limited to 'net/dccp/ccids/ccid3.h')
-rw-r--r-- | net/dccp/ccids/ccid3.h | 19 |
1 files changed, 14 insertions, 5 deletions
diff --git a/net/dccp/ccids/ccid3.h b/net/dccp/ccids/ccid3.h index 0cfcfff8f5fb..92a5e1688a07 100644 --- a/net/dccp/ccids/ccid3.h +++ b/net/dccp/ccids/ccid3.h | |||
@@ -47,12 +47,23 @@ | |||
47 | /* Two seconds as per RFC 3448 4.2 */ | 47 | /* Two seconds as per RFC 3448 4.2 */ |
48 | #define TFRC_INITIAL_TIMEOUT (2 * USEC_PER_SEC) | 48 | #define TFRC_INITIAL_TIMEOUT (2 * USEC_PER_SEC) |
49 | 49 | ||
50 | /* In usecs - half the scheduling granularity as per RFC3448 4.6 */ | ||
51 | #define TFRC_OPSYS_HALF_TIME_GRAN (USEC_PER_SEC / (2 * HZ)) | ||
52 | |||
53 | /* Parameter t_mbi from [RFC 3448, 4.3]: backoff interval in seconds */ | 50 | /* Parameter t_mbi from [RFC 3448, 4.3]: backoff interval in seconds */ |
54 | #define TFRC_T_MBI 64 | 51 | #define TFRC_T_MBI 64 |
55 | 52 | ||
53 | /* | ||
54 | * The t_delta parameter (RFC 3448, 4.6): delays of less than %USEC_PER_MSEC are | ||
55 | * rounded down to 0, since sk_reset_timer() here uses millisecond granularity. | ||
56 | * Hence we can use a constant t_delta = %USEC_PER_MSEC when HZ >= 500. A coarse | ||
57 | * resolution of HZ < 500 means that the error is below one timer tick (t_gran) | ||
58 | * when using the constant t_delta = t_gran / 2 = %USEC_PER_SEC / (2 * HZ). | ||
59 | */ | ||
60 | #if (HZ >= 500) | ||
61 | # define TFRC_T_DELTA USEC_PER_MSEC | ||
62 | #else | ||
63 | # define TFRC_T_DELTA (USEC_PER_SEC / (2 * HZ)) | ||
64 | #warning Coarse CONFIG_HZ resolution -- higher value recommended for TFRC. | ||
65 | #endif | ||
66 | |||
56 | enum ccid3_options { | 67 | enum ccid3_options { |
57 | TFRC_OPT_LOSS_EVENT_RATE = 192, | 68 | TFRC_OPT_LOSS_EVENT_RATE = 192, |
58 | TFRC_OPT_LOSS_INTERVALS = 193, | 69 | TFRC_OPT_LOSS_INTERVALS = 193, |
@@ -92,7 +103,6 @@ enum ccid3_hc_tx_states { | |||
92 | * @no_feedback_timer - Handle to no feedback timer | 103 | * @no_feedback_timer - Handle to no feedback timer |
93 | * @t_ld - Time last doubled during slow start | 104 | * @t_ld - Time last doubled during slow start |
94 | * @t_nom - Nominal send time of next packet | 105 | * @t_nom - Nominal send time of next packet |
95 | * @delta - Send timer delta (RFC 3448, 4.6) in usecs | ||
96 | * @hist - Packet history | 106 | * @hist - Packet history |
97 | * @options_received - Parsed set of retrieved options | 107 | * @options_received - Parsed set of retrieved options |
98 | */ | 108 | */ |
@@ -111,7 +121,6 @@ struct ccid3_hc_tx_sock { | |||
111 | struct timer_list no_feedback_timer; | 121 | struct timer_list no_feedback_timer; |
112 | ktime_t t_ld; | 122 | ktime_t t_ld; |
113 | ktime_t t_nom; | 123 | ktime_t t_nom; |
114 | u32 delta; | ||
115 | struct tfrc_tx_hist_entry *hist; | 124 | struct tfrc_tx_hist_entry *hist; |
116 | struct ccid3_options_received options_received; | 125 | struct ccid3_options_received options_received; |
117 | }; | 126 | }; |