ffmpeg filter_complex concat never completes - ffmpeg

Question

I am trying to concatenate mp4 files into a single file. I am attempting to use the concat filter directly for reasons independent from this question (so -i list.txt is not a valid solution). All video files in question are between 4 and 20 minutes in length.
What I am executing:
% ffmpeg -i f01.mp4 -i f02.mp4 -filter_complex "[0:v][0:a][1:v][1:a]concat=n=2:v=1:a=1[v][a]" -map "[v]" -map "[a]" output.mp4
This is my understanding of the form of this command from references such as FFmpeg Filters Documentation: concat and Concatenate Videos Together Using FFMPEG!.
What happens:
ffmpeg gives a lot of output as it checks the metadata for each stream and then begins processing. After a short duration (several seconds to a minute or so, seems to be prortional to the duration of the first video) I start seeing messages like this:
More than 1000 frames duplicated
More than 10000 frames duplicated 512kB time=00:00:00.12 bitrate=32771.0kbits/s dup=33365 drop=0 speed=0.00449x
More than 100000 frames duplicated 1280kB time=00:00:00.17 bitrate=61442.1kbits/s dup=66730 drop=0 speed=0.00272x
... and then the process never completes. If I leave my computer running for 24 hours, ffmpeg is still using max available CPU (200-300%). The output file is 48 bytes in length.
Note: the inputs are only a few minutes each, so individually re-encoding the inputs would take only a few minutes each.
Note: if I change the command to use f01.mp4 for both source 0 and source 1, the command completes as expected in under 5 minutes:
% # example that works:
% # note, however, that this just repeats the same source twice
% ffmpeg -i f01.mp4 -i f01.mp4 -filter_complex "[0:v][0:a][1:v][1:a]concat=n=2:v=1:a=1[v][a]" -map "[v]" -map "[a]" output.mp4
Full output:
Here is the complete output of a run in case it's helpful:
% ffmpeg -i f01.mp4 -i f02.mp4 -filter_complex "[0:v][0:a][1:v][1:a]concat=n=2:v=1:a=1[v][a]" -map "[v]" -map "[a]" output.mp4
ffmpeg version 4.3 Copyright (c) 2000-2020 the FFmpeg developers
built with Apple clang version 11.0.3 (clang-1103.0.32.62)
configuration: --prefix=/usr/local/Cellar/ffmpeg/4.3_2 --enable-shared --enable-pthreads --enable-version3 --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libbluray --enable-libdav1d --enable-libmp3lame --enable-libopus --enable-librav1e --enable-librubberband --enable-libsnappy --enable-libsrt --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librtmp --enable-libspeex --enable-libsoxr --enable-videotoolbox --disable-libjack --disable-indev=jack
libavutil 56. 51.100 / 56. 51.100
libavcodec 58. 91.100 / 58. 91.100
libavformat 58. 45.100 / 58. 45.100
libavdevice 58. 10.100 / 58. 10.100
libavfilter 7. 85.100 / 7. 85.100
libavresample 4. 0. 0 / 4. 0. 0
libswscale 5. 7.100 / 5. 7.100
libswresample 3. 7.100 / 3. 7.100
libpostproc 55. 7.100 / 55. 7.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'f01.mp4':
Metadata:
major_brand : mp42
minor_version : 512
compatible_brands: isomiso2avc1mp41
creation_time : 2020-06-04T21:34:26.000000Z
encoder : HandBrake 1.3.2 2020050300
Duration: 00:04:14.66, start: 0.000000, bitrate: 525 kb/s
Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv, smpte170m/smpte170m/bt709), 708x478 [SAR 8:9 DAR 944:717], 366 kb/s, 29.97 fps, 29.97 tbr, 90k tbn, 180k tbc (default)
Metadata:
creation_time : 2020-06-04T21:34:26.000000Z
handler_name : VideoHandler
Stream #0:1(jpn): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 149 kb/s (default)
Metadata:
creation_time : 2020-06-04T21:34:26.000000Z
handler_name : Stereo
Input #1, mov,mp4,m4a,3gp,3g2,mj2, from 'f02.mp4':
Metadata:
major_brand : mp42
minor_version : 512
compatible_brands: isomiso2avc1mp41
creation_time : 2020-06-04T21:38:15.000000Z
encoder : HandBrake 1.3.2 2020050300
Duration: 00:06:30.95, start: 0.000000, bitrate: 1328 kb/s
Stream #1:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv, smpte170m/smpte170m/bt709), 708x478 [SAR 8:9 DAR 944:717], 1179 kb/s, 29.97 fps, 30 tbr, 90k tbn, 180k tbc (default)
Metadata:
creation_time : 2020-06-04T21:38:15.000000Z
handler_name : VideoHandler
Stream #1:1(jpn): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 140 kb/s (default)
Metadata:
creation_time : 2020-06-04T21:38:15.000000Z
handler_name : Stereo
File 'output.mp4' already exists. Overwrite? [y/N] y
Stream mapping:
Stream #0:0 (h264) -> concat:in0:v0
Stream #0:1 (aac) -> concat:in0:a0
Stream #1:0 (h264) -> concat:in1:v0
Stream #1:1 (aac) -> concat:in1:a0
concat:out:v0 -> Stream #0:0 (libx264)
concat:out:a0 -> Stream #0:1 (aac)
Press [q] to stop, [?] for help
[mp4 # 0x7ff130014000] Frame rate very high for a muxer not efficiently supporting it.
Please consider specifying a lower framerate, a different muxer or -vsync 2
[libx264 # 0x7ff130021200] using SAR=8/9
[libx264 # 0x7ff130021200] MB rate (1350000000) > level limit (16711680)
[libx264 # 0x7ff130021200] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 # 0x7ff130021200] profile High, level 6.2, 4:2:0, 8-bit
[libx264 # 0x7ff130021200] 264 - core 160 r3011 cde9a93 - H.264/MPEG-4 AVC codec - Copyleft 2003-2020 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=6 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'output.mp4':
Metadata:
major_brand : mp42
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf58.45.100
Stream #0:0: Video: h264 (libx264) (avc1 / 0x31637661), yuv420p(progressive), 708x478 [SAR 8:9 DAR 944:717], q=-1--1, 1000k tbn, 1000k tbc (default)
Metadata:
encoder : Lavc58.91.100 libx264
Side data:
cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A
Stream #0:1: Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 128 kb/s (default)
Metadata:
encoder : Lavc58.91.100 aac
More than 1000 frames duplicated
More than 10000 frames duplicated 512kB time=00:00:00.12 bitrate=32771.0kbits/s dup=33365 drop=0 speed=0.00449x
More than 100000 frames duplicated 1280kB time=00:00:00.17 bitrate=61442.1kbits/s dup=66730 drop=0 speed=0.00272x
frame=667333 fps=1079 q=33.0 size= 14848kB time=00:00:00.76 bitrate=158379.2kbits/s dup=667312 drop=0 speed=0.00124x

Answer 1

As the warning in the log says, Frame rate very high for a muxer not efficiently supporting it. This happens because concat unsets the stream frame rate, which happens when the tbr of all video streams don't match. ffmpeg will then use the time_base as the effective frame rate and then duplicate frames to match that frame rate. In this case, it's 90000, so this effectively explodes resource use and slows down the process.
Add -vsync 2 to avoid frame duplication.
ffmpeg -i f01.mp4 -i f02.mp4 -filter_complex "[0:v][0:a][1:v][1:a]concat=n=2:v=1:a=1[v][a]" -vsync 2 -map "[v]" -map "[a]" output.mp4