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doc/AMR-EFR-performance: new article
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 20 May 2024 21:53:11 +0000
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cd1f0fa936cc doc/AMR-EFR-performance: new article
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1 Performance of libgsmefr and libtwamr, compared to competition
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2 ==============================================================
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3
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4 Both libgsmefr and libtwamr are based on reference C code from ETSI/3GPP:
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5 libgsmefr is a librified version of GSM 06.53 reference code, and libtwamr is a
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6 librified version of TS 26.073 reference code. Both of those reference sources
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7 were officially presented as simulations, not as production code for running on
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8 general-purpose x86 servers implementing transcoding MGW functionality, and the
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9 code is extremely inefficient.
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10
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11 The problem of poor performance (taking too much CPU percentage per transcoded
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12 call) would not be so acute if Themyscira GSM codec libraries existed in a
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13 vacuum without competition - but we do have a competitor in the form of
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14 libopencore-amrnb, part of the slightly larger opencore-amr package. Just like
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15 libtwamr, that library is a derivative work based on the original AMR C code
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16 from 3GPP, but by a more circuitous route: first PacketVideo created an
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17 implementation of AMR for Android based on 3GPP reference code, then the
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18 creators of opencore-amr took PV's Android implementation (named OpenCORE) and
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19 ported it from Android to standard Unix/Linux userspace.
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20
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21 Aside from its peculiar genealogy, there are practical problems with
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22 libopencore-amrnb that made it unattractive for Themyscira Wireless, and thus
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23 created the impetus for producing the present alternative:
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24
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25 * That library implements only AMR and not EFR, whereas for ThemWi good support
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26 for EFR is a higher priority than good (or any) support for AMR.
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27
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28 * The idea of reshuffling bits in the manner of our gsm-amr2efr and gsm-efr2amr
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29 hack-utilities (see AMR-EFR-conversion article) in order to implement "poor
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30 man's EFR" via libopencore-amrnb was considered, but then rejected on
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31 philosophical and aesthetic grounds: it feels philosophically wrong to
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32 reshuffle bits in the application wrapper only to have internal code within
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33 the library shuffle them back into the natural order of codec parameters.
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34
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35 * Like any AMR-and-not-EFR library, libopencore-amrnb cannot grok EFR SID
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36 frames. Therefore, an implementation of "poor man's EFR" using that library
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37 would have to operate with DTXu disabled, needlessly burning battery capacity
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38 in the MS.
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39
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40 * Even for "pure" AMR and not EFR, libopencore-amrnb exhibits one serious
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41 defect: it omits the in-band homing feature in both the encoder and the
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42 decoder, even though this feature is mandatory per 3GPP specs. (It is not
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43 clear if this removal of homing functions was the work of PacketVideo in the
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44 world of Android, or if it happened at the time of subsequent opencore-amr
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45 port to standard Unix/Linux userspace.) Furthermore, because the interface
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46 wrapper module of libopencore-amrnb does not expose any state reset functions,
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47 it is impossible to implement the homing feature externally to the library.
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48 This lack of homing makes it impossible to test libopencore-amrnb against the
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49 full set of 3GPP test sequences for AMR.
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50
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51 In the present time, as of this release of Themyscira gsm-codec-lib, libgsmefr
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52 is intended for production use while libtwamr is intended for lab use only,
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53 matching test sequences and modeling bit-exact operation of other people's
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54 network elements in reverse engineering workflows. Based on this difference in
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55 applicability to production, more work has been put into improving performance
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56 of libgsmefr compared to libtwamr. Libgsmefr version 1.1.0 exhibits
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57 approximately 2x performance improvement over the original version, but it is
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58 still significantly slower than libopencore-amrnb.
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59
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60 The following performance numbers were obtained on Mother Mychaela's development
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61 laptop, Intel Core2 Duo P8600 CPU @ 2.40 GHz, running 32-bit version of
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62 Slackware, i.e., running only i686 code and not x86_64 despite the CPU having
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63 the 'lm' feature. All tests involve encoding TEST4.INP sequence from GSM 06.54
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64 set (301 frames in total, 6.020 s audio duration), then decoding the output of
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65 the encoder run.
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66
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67 Implementation Encoding time Decoding time
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68 ---------------------------------------------
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69 libgsmefr 183 ms 23 ms
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70 libtwamr 359 ms 52 ms
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71 opencore-amr 53 ms 13 ms
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72 libgsmfr2 (FR) 13 ms 6 ms
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73
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74 FRv1 encoding and decoding of the same TEST4.INP was included in the test in
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75 order to convince ourselves that FRv1 is not in any need of performance
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76 optimization. In a network deployment that generally prefers EFR over FRv1,
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77 almost all calls between a local GSM MS and the outside world (PSTN) will
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78 involve the EFR transcoder rather than FRv1. If on rare occasion the network
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79 selects FRv1 instead of EFR because the subscriber is using a super-old
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80 non-EFR-capable handset (or has artificially restricted the speech version list
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81 to FRv1 only), there will be some CPU cycles saved "for free" by using the much
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82 less computationally expensive FRv1 codec - but CPU capacity planning for the
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83 transcoding MGW has to be based on EFR performance numbers rather than FRv1.
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84
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85 In terms of EFR or MR122 encoder and decoder CPU cycle demand, libtwamr is
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86 currently the worst performer; the first version of libgsmefr exhibited the
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87 same poor performance. The current version of libgsmefr exhibits somewhat
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88 better performance because most saturation arithmetic "basic ops" have been
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89 inlined (eliminating the use of a function call for every elementary operation),
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90 and the implementation code of those newly-inlined functions has been
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91 streamlined for performance, as much as possible with immediately obvious
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92 programming methods. The result is about 2x improvement over the original -
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93 but sadly, still significantly slower than libopencore-amrnb on the same
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94 machine with the same 32-bit execution mode and ABI.
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95
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96 Given that we (Themyscira Wireless) have a ton of other work tasks in our queue
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97 besides the present problem of performance-optimizing libgsmefr to match
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98 opencore-amr, this problem is currently shelved. We are releasing our libraries
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99 as they currently stand, with the ~2x improvement in libgsmefr over the original
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100 but still not matching opencore-amr, and leaving the problem of further
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101 performance improvement to be addressed later, after other work of higher
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102 priority.