[dm-crypt] multiple crypt devices not using multiple CPUs

Brian J. Murrell brian at interlinx.bc.ca
Sun Dec 16 22:04:05 CET 2012


Hi,

I guess I must be missing something.  I had understood that if one had
multiple block devices being crypted by dm-crypt, if they were being
operated on at the same time, their parallel crypto operations would
utilize different CPUs.

So I just did the following:

# lvcreate -L10G -n t1 vg
# cryptsetup --verbose --verify-passphrase luksFormat /dev/vg/t1
# cryptsetup luksOpen /dev/vg/t1 t1-use
# dd of=/dev/null if=/dev/mapper/t1-use bs=1M count=1000
1000+0 records in
1000+0 records out
1048576000 bytes (1.0 GB) copied, 12.1207 s, 86.5 MB/s

observe (a series of) "kworker(s)" each using near 100% of a core to
carry out the crypto.  I say a series of kworkers since it appears to
almost be a "relay" of them, each handing the work of to another and
going away.  But all in all there is only every near 1 full core's worth
running.

So now...

# lvcreate -L10G -n t2 vg
# cryptsetup --verbose --verify-passphrase luksFormat /dev/vg/t2
# cryptsetup luksOpen /dev/vg/t2 t2-use
# for ((x=1; x<3; x++)); do
>     dd of=/dev/null if=/dev/mapper/t$x-use bs=1M count=1000 &
> done
[1] 7369
[2] 7370
# 1000+0 records in
1000+0 records out
1048576000 bytes (1.0 GB) copied, 27.62 s, 38.0 MB/s
1000+0 records in
1000+0 records out
1048576000 bytes (1.0 GB) copied, 27.8015 s, 37.7 MB/s

Here I observe two kworker threads each near 50% of a core.  Clearly
they are both using the same core.  But why?  As as an aside, aggregate
throughput of the two threads is quite a bit less than a single thread.

But what I really want to understand is why crypto kworkers are not
being scheduled to different, idle cores.

Cheers,
b.

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