Comparing Performance 100 Hz versus 1000 Hz Clock Tick
It is perspicuous that a system running with a clock tick of 1000 Hz suffers from a
higher system overhead than one running with a 100 Hz tick: all clock tick
related activities must run ten times as often.
But how big is the impact on system performance really?
Theoretical Expectations
On a MPC860-T at 50 MHz the overhead to handle a trivial interrupt in Linux is approx. 50
microseconds.
If the clock tick happens 100 Times per second, this is 50 * 100 us
or 5 milliseconds or 0.5% of the total available peformance.
With a clock tick of 1 kHz we have 50 * 1000 us = 50 milliseconds or
5% of the total available peformance.
Which means that the increase of the clock tick alone should result
in a performance drop of approximately 4.5%.
Note: assume you run exactly the same code on a 400 MHz processor;
here the interrupt processing time will be approx. 10 us. This means
an overhead of 1 ms or 0.1% at a tick of 100 Hz and ome of 10ms or 1%
at 1kHz - which means that the impact of the increased clock tick is
just 0.9%!
Benchmark Results
Here is the results of running the
lmbench benchmark on a TQM860L board
(MPC860-T processor at 50 MHz, 16 MB SDRAM, 4 GB ATA disk on the PCMCIA controller).
5 test runs were made each with a clock tick of 100 Hz (system name
"tqm860l")
and with a tick of 1000 Hz (system name
"tqm880l1k"):
L M B E N C H 3 . 0 S U M M A R Y
------------------------------------
(Alpha software, do not distribute)
Basic system parameters
------------------------------------------------------------------------------
Host OS Description Mhz tlb cache mem scal
pages line par load
bytes
--------- ------------- ----------------------- ---- ----- ----- ------ ----
tqm860l Linux 2.4.25- powerpc-linux-gnu 50 32 16 1.0100 1
tqm860l Linux 2.4.25- powerpc-linux-gnu 50 32 16 1.0200 1
tqm860l Linux 2.4.25- powerpc-linux-gnu 50 32 16 1.0200 1
tqm860l Linux 2.4.25- powerpc-linux-gnu 50 32 16 1.0200 1
tqm860l Linux 2.4.25- powerpc-linux-gnu 50 32 16 1.0100 1
============================================================================
tqm860l Tick 100 Avrg: 50 32 16 1.0200 1
============================================================================
tqm860l1k Linux 2.4.25- powerpc-linux-gnu 50 32 16 1.0100 1
tqm860l1k Linux 2.4.25- powerpc-linux-gnu 50 32 16 1.0100 1
tqm860l1k Linux 2.4.25- powerpc-linux-gnu 50 32 16 1.0000 1
tqm860l1k Linux 2.4.25- powerpc-linux-gnu 50 32 16 1.0000 1
tqm860l1k Linux 2.4.25- powerpc-linux-gnu 50 32 16 1.0200 1
============================================================================
tqm860l Tick 1000 Avrg: 50 32 16 1.0100 1
+-0 +-0 +-0 -1.0 +-0 %
============================================================================
Processor, Processes - times in microseconds - smaller is better
------------------------------------------------------------------------------
Host OS Mhz null null open slct sig sig fork exec sh
call I/O stat clos TCP inst hndl proc proc proc
--------- ------------- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ----
tqm860l Linux 2.4.25- 50 6.19 10.4 79.9 154. 490. 23.8 71.5 6930 30.K 105K
tqm860l Linux 2.4.25- 50 6.20 10.4 80.4 155. 510. 23.9 70.8 6904 30.K 105K
tqm860l Linux 2.4.25- 50 6.20 10.4 77.9 154. 489. 24.0 70.5 6928 30.K 105K
tqm860l Linux 2.4.25- 50 6.20 10.4 79.8 153. 507. 23.9 69.1 6956 30.K 105K
tqm860l Linux 2.4.25- 50 6.20 10.6 79.7 153. 485. 23.9 69.6 6893 31.K 106K
============================================================================
tqm860l Tick 100 Avrg: 50 6.20 10.4 79.6 154 496 23.9 70.3 6922 30 K 105K
============================================================================
tqm860l1k Linux 2.4.25- 50 5.83 10.8 86.2 128. 512. 24.5 71.1 7230 32.K 111K
tqm860l1k Linux 2.4.25- 50 5.82 11.0 85.3 163. 507. 24.6 70.5 7188 32.K 111K
tqm860l1k Linux 2.4.25- 50 5.83 10.8 89.5 166. 547. 24.5 70.6 7258 32.K 111K
tqm860l1k Linux 2.4.25- 50 5.83 10.7 84.8 164. 507. 24.5 71.0 7243 32.K 111K
tqm860l1k Linux 2.4.25- 50 5.81 10.8 84.1 161. 511. 24.5 70.7 7232 32.K 111K
============================================================================
tqm860l Tick 1000 Avrg: 50 5.82 10.8 86.0 156 517 24.5 70.8 7230 32 K 111K
+-0 -6.1 +3.9 +8.0 +1.3 +4.2 +2.5 +0.7 +4.5 +6.7 +5.7 %
============================================================================
Basic integer operations - times in nanoseconds - smaller is better
-------------------------------------------------------------------
Host OS intgr intgr intgr intgr intgr
bit add mul div mod
--------- ------------- ------ ------ ------ ------ ------
tqm860l Linux 2.4.25- 20.6 20.5 42.3 161.0 260.4
tqm860l Linux 2.4.25- 20.6 20.5 42.3 161.0 260.5
tqm860l Linux 2.4.25- 20.6 20.4 42.4 161.0 260.6
tqm860l Linux 2.4.25- 20.6 20.4 42.0 161.0 260.6
tqm860l Linux 2.4.25- 20.6 20.5 42.3 161.0 260.4
============================================================================
tqm860l Tick 100 Avrg: 20.6 20.4 42.3 161.0 260.5
============================================================================
tqm860l1k Linux 2.4.25- 21.2 21.0 42.7 164.6 268.6
tqm860l1k Linux 2.4.25- 21.2 21.0 42.7 163.9 268.6
tqm860l1k Linux 2.4.25- 21.1 21.0 42.7 163.9 268.6
tqm860l1k Linux 2.4.25- 21.0 21.0 42.7 163.9 270.2
tqm860l1k Linux 2.4.25- 21.2 21.0 42.7 164.6 268.6
============================================================================
tqm860l Tick 1000 Avrg: 21.1 21.0 42.7 164.2 268.9
+2.4 +2.9 +1.0 +2.0 +3.2 %
============================================================================
Basic float operations - times in nanoseconds - smaller is better
-----------------------------------------------------------------
Host OS float float float float
add mul div bogo
--------- ------------- ------ ------ ------ ------
tqm860l Linux 2.4.25- 4490.9 4114.0 8295.0 22.0K
tqm860l Linux 2.4.25- 4490.9 4114.7 8295.0 22.0K
tqm860l Linux 2.4.25- 4490.9 4114.0 8295.0 22.0K
tqm860l Linux 2.4.25- 4490.9 4114.0 8295.0 22.0K
tqm860l Linux 2.4.25- 4490.9 4114.0 8271.4 22.0K
============================================================================
tqm860l Tick 100 Avrg: 4490.9 4114.1 8290.3 22.0K
============================================================================
tqm860l1k Linux 2.4.25- 4609.3 4235.6 8696.7 22.8K
tqm860l1k Linux 2.4.25- 4610.1 4235.6 8646.7 22.8K
tqm860l1k Linux 2.4.25- 4636.4 4234.9 8648.3 22.8K
tqm860l1k Linux 2.4.25- 4610.1 4235.6 8700.0 22.8K
tqm860l1k Linux 2.4.25- 4627.1 4242.2 8645.0 22.8K
============================================================================
tqm860l Tick 1000 Avrg: 4618.6 4236.8 8667.3 22.8K
+2.8 +3.0 +4.6 +3.6 %
============================================================================
Basic double operations - times in nanoseconds - smaller is better
------------------------------------------------------------------
Host OS double double double double
add mul div bogo
--------- ------------- ------ ------ ------ ------
tqm860l Linux 2.4.25- 6372.7 8488.2 33.5K 54.8K
tqm860l Linux 2.4.25- 6373.9 8429.7 33.5K 54.5K
tqm860l Linux 2.4.25- 6373.9 8488.2 33.5K 54.5K
tqm860l Linux 2.4.25- 6373.9 8488.2 33.5K 54.8K
tqm860l Linux 2.4.25- 6373.9 8420.6 33.5K 54.4K
============================================================================
tqm860l Tick 100 Avrg: 6373.7 8463.0 33.5K 54.6K
============================================================================
tqm860l1k Linux 2.4.25- 6563.4 8812.5 34.5K 57.2K
tqm860l1k Linux 2.4.25- 6600.2 8812.5 34.5K 57.2K
tqm860l1k Linux 2.4.25- 6597.9 8814.1 34.5K 57.3K
tqm860l1k Linux 2.4.25- 6604.1 8810.9 34.5K 57.2K
tqm860l1k Linux 2.4.25- 6565.8 8810.9 34.5K 57.2K
============================================================================
tqm860l Tick 1000 Avrg: 6586.3 8812.2 34.5K 57.2K
+3.3 +4.1 +3.0 +4.8 %
============================================================================
Context switching - times in microseconds - smaller is better
-------------------------------------------------------------------------
Host OS 2p/0K 2p/16K 2p/64K 8p/16K 8p/64K 16p/16K 16p/64K
ctxsw ctxsw ctxsw ctxsw ctxsw ctxsw ctxsw
--------- ------------- ------ ------ ------ ------ ------ ------- -------
tqm860l Linux 2.4.25- 34.8 66.0 62.8 81.3 77.4 81.7 78.6
tqm860l Linux 2.4.25- 23.2 62.5 78.0 82.2 82.1 83.3 80.6
tqm860l Linux 2.4.25- 30.1 72.9 74.0 89.0 76.6 82.5 80.4
tqm860l Linux 2.4.25- 25.9 73.2 66.8 88.7 80.9 88.1 85.4
tqm860l Linux 2.4.25- 32.6 59.5 76.8 77.4 85.5 81.3 86.6
============================================================================
tqm860l Tick 100 Avrg: 29.3 66.8 71.7 83.7 80.5 83.4 82.3
============================================================================
tqm860l1k Linux 2.4.25- 23.4 62.3 57.8 88.3 73.1 93.6 78.1
tqm860l1k Linux 2.4.25- 25.0 61.1 53.2 91.0 70.8 90.2 76.6
tqm860l1k Linux 2.4.25- 23.2 64.4 55.8 87.3 76.1 87.0 76.4
tqm860l1k Linux 2.4.25- 24.8 65.9 55.2 84.9 72.9 93.0 77.3
tqm860l1k Linux 2.4.25- 23.3 65.2 55.8 87.9 76.5 90.1 77.4
============================================================================
tqm860l Tick 1000 Avrg: 23.9 63.8 55.6 87.9 73.9 90.8 77.2
-18.4 -4.5 -22.4 +5.0 -8.2 +8.9 -6.2 %
============================================================================
*Local* Communication latencies in microseconds - smaller is better
---------------------------------------------------------------------
Host OS 2p/0K Pipe AF UDP RPC/ TCP RPC/ TCP
ctxsw UNIX UDP TCP conn
--------- ------------- ----- ----- ---- ----- ----- ----- ----- ----
tqm860l Linux 2.4.25- 34.8 143.2 361. 694.2 1430. 998.5 1922. 2941
tqm860l Linux 2.4.25- 23.2 148.7 340. 731.9 1430. 998.2 1917. 2975
tqm860l Linux 2.4.25- 30.1 152.6 350. 723.4 1446. 998.5 1906. 2984
tqm860l Linux 2.4.25- 25.9 145.9 343. 694.5 1428. 995.2 1940. 2963
tqm860l Linux 2.4.25- 32.6 142.2 344. 721.3 1434. 997.3 1904. 2987
============================================================================
tqm860l Tick 100 Avrg: 29.3 146.5 348 713.1 1434. 997.5 1918. 2970
============================================================================
tqm860l1k Linux 2.4.25- 23.4 143.4 374. 712.2 1500. 1040. 2002. 3121
tqm860l1k Linux 2.4.25- 25.0 148.1 354. 729.3 1500. 1031. 2006. 3114
tqm860l1k Linux 2.4.25- 23.2 147.2 358. 730.5 1502. 1028. 2002. 3124
tqm860l1k Linux 2.4.25- 24.8 142.4 355. 724.1 1504. 1017. 2000. 3126
tqm860l1k Linux 2.4.25- 23.3 150.6 363. 712.6 1498. 1034. 2000. 3144
============================================================================
tqm860l Tick 1000 Avrg: 23.9 146.3 361. 721.8 1501. 1030. 2002. 3126
-18.4 -0.1 +3.7 +1.2 +4.7 +3.3 +4.4 +5.3 %
============================================================================
File & VM system latencies in microseconds - smaller is better
-------------------------------------------------------------------------------
Host OS 0K File 10K File Mmap Prot Page 100fd
Create Delete Create Delete Latency Fault Fault selct
--------- ------------- ------ ------ ------ ------ ------- ----- ------- -----
tqm860l Linux 2.4.25- 708.7 711.7 3164.6 1251.6 2029.0 20.2 40.1 293.4
tqm860l Linux 2.4.25- 715.8 728.9 3205.1 1257.9 2029.0 15.9 40.1 311.9
tqm860l Linux 2.4.25- 714.3 723.6 3194.9 1265.8 2041.0 16.5 40.1 293.4
tqm860l Linux 2.4.25- 709.7 713.8 3164.6 1251.6 2070.0 18.2 40.7 293.3
tqm860l Linux 2.4.25- 717.4 740.7 3164.6 1259.4 2050.0 18.8 40.1 290.2
============================================================================
tqm860l Tick 100 Avrg: 713.2 723.7 3178.8 1257.3 2043.8 17.9 40.2 296.4
============================================================================
tqm860l1k Linux 2.4.25- 777.0 788.0 3424.7 1351.4 2126.0 18.9 42.4 304.9
tqm860l1k Linux 2.4.25- 781.2 783.7 3436.4 1355.0 2158.0 20.9 42.4 305.0
tqm860l1k Linux 2.4.25- 784.9 791.1 3448.3 1360.5 2160.0 20.1 43.0 303.1
tqm860l1k Linux 2.4.25- 781.2 788.0 3436.4 1360.5 2136.0 20.1 42.6 302.9
tqm860l1k Linux 2.4.25- 777.6 785.5 3460.2 1345.9 2173.0 20.1 43.1 304.2
============================================================================
tqm860l Tick 1000 Avrg: 780.4 787.3 3441.2 1354.7 2150.6 20.0 42.7 304.0
+9.4 +8.8 +8.2 +7.8 +5.2 +11.7 +6.2 +2.6 %
============================================================================
*Local* Communication bandwidths in MB/s - bigger is better
-----------------------------------------------------------------------------
Host OS Pipe AF TCP File Mmap Bcopy Bcopy Mem Mem
UNIX reread reread (libc) (hand) read write
--------- ------------- ---- ---- ---- ------ ------ ------ ------ ---- -----
tqm860l Linux 2.4.25- 9.82 10.4 8.12 15.3 43.0 26.2 26.7 43.1 43.2
tqm860l Linux 2.4.25- 9.83 10.4 8.13 15.3 43.0 26.2 26.7 43.1 43.2
tqm860l Linux 2.4.25- 9.82 10.4 8.13 15.3 43.0 26.2 26.7 43.1 43.2
tqm860l Linux 2.4.25- 9.82 10.4 8.11 15.3 43.0 26.2 26.7 43.1 43.2
tqm860l Linux 2.4.25- 9.82 10.4 8.12 15.3 43.0 26.2 26.7 43.1 43.1
============================================================================
tqm860l Tick 100 Avrg: 9.82 10.4 8.12 15.3 43.0 26.2 26.7 43.1 43.2
============================================================================
tqm860l1k Linux 2.4.25- 9.35 9.87 7.69 14.6 41.2 25.1 25.1 41.4 41.2
tqm860l1k Linux 2.4.25- 9.35 9.84 7.70 14.6 41.2 25.1 25.1 41.4 41.2
tqm860l1k Linux 2.4.25- 9.35 9.84 7.68 14.6 41.2 25.1 25.1 41.4 41.2
tqm860l1k Linux 2.4.25- 9.35 9.82 7.69 14.6 41.2 25.1 25.1 41.4 41.2
tqm860l1k Linux 2.4.25- 9.32 9.82 7.68 14.6 41.2 25.1 25.1 41.4 41.2
============================================================================
tqm860l Tick 1000 Avrg: 9.34 9.84 7.67 14.6 41.2 25.1 25.1 41.4 41.2
-4.9 -5.4 -5.5 -4.6 -4.2 -4.2 -6.0 -3.9 -4.6 %
============================================================================
Memory latencies in nanoseconds - smaller is better
(WARNING - may not be correct, check graphs)
------------------------------------------------------------------------------
Host OS Mhz L1 $ L2 $ Main mem Rand mem Guesses
--------- ------------- --- ---- ---- -------- -------- -------
tqm860l Linux 2.4.25- 50 42.4 183.2 235.1 1372.2
tqm860l Linux 2.4.25- 50 42.3 183.3 235.1 1371.4
tqm860l Linux 2.4.25- 50 42.3 183.3 235.1 1368.1
tqm860l Linux 2.4.25- 50 42.3 183.2 235.1 1371.3
tqm860l Linux 2.4.25- 50 42.4 183.3 235.1 1370.5
============================================================================
tqm860l Tick 100 Avrg: 50 42.3 183.3 235.1 1370.7
============================================================================
tqm860l1k Linux 2.4.25- 50 43.2 200.2 243.5 1441.7 No L2 cache?
tqm860l1k Linux 2.4.25- 50 43.2 200.1 243.6 1441.1 No L2 cache?
tqm860l1k Linux 2.4.25- 50 43.2 200.2 243.6 1442.1 No L2 cache?
tqm860l1k Linux 2.4.25- 50 43.2 200.1 243.6 1442.3 No L2 cache?
tqm860l1k Linux 2.4.25- 50 43.2 200.2 243.6 1443.6 No L2 cache?
============================================================================
tqm860l Tick 1000 Avrg: 50 43.2 200.2 243.6 1442.2
+-0 +2.1 +9.2 +3.6 +5.2 %
============================================================================
Interpretation of the Results
Plain CPU performance related tests ("Basic integer operations",
"Basic float operations" resp. "Basic double operations")
run approx. 4% slower,
which is a goot match with our expectations.
For other tests it is more difficult to interpret the results; for
example it is not immediately clear how much influence goes back to
the fact that the time measurement happens at 10 times better
accuracy at the higher clock, so rounding / cutoff errors may cause a shift of
the results.
Most tests show a performance degradadion of 5...6%, in some cases
near 10%.
Especially big differences can be seen with the context switching
tests - in some cases the 1 kHz system is nearly 20% faster, in
other cases it is 10% slower.
Here it is impossible to come up with clean
statements about the effect that can be expected for real life
applications.
Summary
The measurents are consistent
with the theoretic expectations. The increased clock tick alone
contributes to the performance degradation by about 5% on low end
systems.
On systems with higher performance the effect is much less
significant.
This probably explains why many developers who never
use low-end embedded processors don't care about
(and usually don't even know of)
these problems in
the 2.6 kernel.
--
WolfgangDenk - 06 Jul 2005
