[dm-devel] [PATCH] dm-crypt: limit the number of allocated pages

[John Stoffel]

>>>>> "Mikulas" == Mikulas Patocka <mpatocka at redhat.com> writes:

Mikulas> On Fri, 18 Aug 2017, John Stoffel wrote:

Mikulas> The limit can't be changed. I expect that no one will run a system with 
Mikulas> such a tight requirements that the memory is used up to the last 2%.
>> 
Mikulas> If the user is near hitting the memory limit, he can just add swap space 
Mikulas> to solve the problem instead of tuning dm-crypt.
>> 
>> My real concern is that we build up too many outstanding BIOs so that
>> they take along time to process.  I'm already seeing some reports
>> about timeouts after 120 seconds due to stacking of dm-crypt with
>> other stuff.
>> 
>> What I'm trying to say is that as memory grows, you need to slow down
>> the growth of the amount of memory that dm-crypt can allocate.  It's
>> more important to scale by the speed of the IO sub-system than the
>> size of memory.
>> 
>> But hey, you really addresed my concern with smaller memory systems
>> already.
>> 
>> John

Mikulas> This patch doesn't restrict the number of in-flight bios. Lowering the 
Mikulas> memory limit has no effect on the number of bios being allocated by some 
Mikulas> other part of the kernel and submitted for dm-crypt.

Good to know.  

Mikulas> Some times ago I made a patch that limits the number of in-flight bios in 
Mikulas> device mapper - see this:
Mikulas> http://people.redhat.com/~mpatocka/patches/kernel/dm-limit-outstanding-bios/

Interesting.  I'll try to find some time to look this over.