Re: [dm-crypt] Poor performane (idle cpu)

[Jakob Sandgren <jakob@southpole.se>]

Hi,

(please keep me on CC since I'm not subscribed yet)

>> I'm using dm-crypt for several mappings with a hardware raid backend. 
>> Using a raw read from the raid device (e.g sda) gives ~250MB/s
>> 
>> But when I read from an encrypted mapping, I just get ~70MB/s. That
>> should be fine if I at least have the kcryptd process using a core
>> at
>> 100%, but that is not the case. Three of my four cores is 99% idle
>> and
>> one core is 50% idle (aprox.).
>
>Which means your core is too slow to support the full 250MB/s
>speed. 
>
>> I have recently upgraded my hardware from an older quadcore system
>> (AMD) to a new Core I7 (860) and expected improved performance and
>> when I did not get that, then did I do some more investegation and
>> found out above. I have also read posts from others having the same
>> problems, but no explanation.
>
>I suspect as the core can support only about 140MB/s encryption
>speed, the accesses get broken. It is well possible that
>if your array would only give 120MB/s it would still have
>that rate encrypted.


This does not make sense to me, I can not understand how a "to fast"
disk could give worse results? Disk requests would get issued at the
speed that decryption can handle(?). I do not understand what a
"broken access" would be. 

Anyway, just to try the theory did I set up a single disk that would
give 120MB sustained read from the unencrypted mapping, but when I
read from the encrypted mapping I still ended up with the low 70MB/s
and a lot of idle cpu. 

Running two reads at the same time (to the same encrypted mapping)
actually increased the combined read rate with ~10% ?! 

To me it seems like there is some serious flaw within kcryptd that
ends up to wait for "something" instead of sending enough requests to
the disks to make sure it has data to decrypt. What do you think?

Best Regards,
Jakob Sandgren
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