Dear Maintainer,

when using arm64 emulation on amd64 host with

    pbuilder build --architecture arm64 --distro trixie

it's incredibly slow to a point where tests fail due to timeouts and
installation of texlive-latex-extra takes tens of minutes.

This problem is present:

* only on Trixie GUESTS (Bookworm, Bullseye, and even Buster guests work
  fine)
* on both Bookworm and Trixie amd64 HOSTS (tested on 2 Bookworm and 1 Trixie
  host)
* only with arm64 emulation, armhf works fine

Furthermore, this has been the case for few months before Trixie
release.

We're talking about 10x to 100x slowdown or more, I'm having really hard times
building arm64 packages rn.

I can do some debugging if you provide instructions / pointers.


Cheers,
Jakub Ružička

Control: tag -1 + moreinfo
Control: severity -1 normal

I've no idea whatsoever how pbuilder executes qemu.  And you don't tell
in the bug report either, - qemu has 2 entirely different modes of
execution, - it is full system emulation and user (process) emulation.

If you're using qemu system emulation, try specifying pauth-impdef
(or not specifying pauth at all) for the cpu features.

Thanks,

/mjt

Control: tag -1 + unreproducible
not qemu.  I guess you can file a bug against arm64 trixie :)

I guess pbuilder means qemu-user package (as I noted before, I never
used pbuilder and don't know how to use it).

I can't confirm this observation, at least not generally.

Some operations has indeed become slower.  For example, I ran openssl
speed benchmark on bookworm guest and trixie guest, here are some
results on x86-64 host (ops/sec, more is better):

Operation    Bookworm  Trixie
md5/16        1496014  981482
md5/64        1382894  967298
md5/256       1158511  883699
md5/1024       526236  469784
md5/8192       107971  130183
md5/16384       58668   72427
sha1/16        794488  571021
sha1/64        601573  466057
sha1/256       362402  309965
sha1/1024      141595  132793
sha1/8192       21644   21597
sha1/16384      11106   10937
...

some operations are slightly faster on trixie though.

However, some other benchmarks are significantly faster on trixie,
for example:

trixie$ 7z b

7-Zip 24.09 (arm64) : Copyright (c) 1999-2024 Igor Pavlov : 2024-11-29
  64-bit arm_v:8-A locale=C.UTF-8 Threads:16 OPEN_MAX:1024, ASM

Compiler:  ver:14.2.0 GCC 14.2.0 : UNALIGNED
Linux : 6.12.38+deb13-amd64 : #1 SMP PREEMPT_DYNAMIC Debian 6.12.38-1
(2025-07-16) : aarch64
PageSize:4KB hwcap:EFFFFFFB:CRC32:SHA1:SHA2:SHA3:SHA512:AES:ASIMD
hwcap2:18007FC77FFF
arm64

1T CPU Freq (MHz):  2297  2308  2330  2336  2327  2328  2328
8T CPU Freq (MHz): 745% 1648   746% 1646
16T CPU Freq (MHz): 1411% 1372   1406% 1118

RAM size:   15695 MB,  # CPU hardware threads:  16
RAM usage:   3559 MB,  # Benchmark threads:     16

                        Compressing  |                  Decompressing
Dict     Speed Usage    R/U Rating  |      Speed Usage    R/U Rating
          KiB/s     %   MIPS   MIPS  |      KiB/s     %   MIPS   MIPS

22:       5552   893    605   5402  |     132019  1412    797  11257
23:       3971   852    475   4046  |      98865  1464    584   8553
24:       5474   917    642   5886  |      85897  1435    525   7537
...

bookworm$ 7z b

7-Zip [64] 16.02 : Copyright (c) 1999-2016 Igor Pavlov : 2016-05-21
p7zip Version 16.02 (locale=C,Utf16=off,HugeFiles=on,64 bits,16 CPUs LE)

LE
CPU Freq: 3764705 - - - - - - - -

RAM size:    4096 MB,  # CPU hardware threads:  16
RAM usage:   3530 MB,  # Benchmark threads:     16

                        Compressing  |                  Decompressing
Dict     Speed Usage    R/U Rating  |      Speed Usage    R/U Rating
          KiB/s     %   MIPS   MIPS  |      KiB/s     %   MIPS   MIPS

22:       4289   967    431   4173  |      64538  1442    382   5504
23:       3807  1017    382   3880  |      62524  1419    381   5410
24:       3872  1027    405   4164  |      65082  1468    389   5713
...

As you can see, decompression on trixie, for dict size of 22, is
2 times faster than on bookworm.  This - most likely - is the result of
code optimization in 7z.

I also tried to compile a few packages in a foreign arm64 chroot on
an x86_64 machine - I don't see dramatic speed difference here, more
like gradual difference due to compiler being "smarter", doing more
optimizations and checking for more cases in the code - maybe 5% the
difference overall.  I observe similar difference in speed when
building the same package on trixie and bookworm natively.

So.. this is an 'unreproducible' for now.

It might be particular source package you're building, or particular
toolchain difference, - I dunno.  It looks like it is not a qemu
problem.

Thanks,

/mjt