On 2/27/23 17:33, Sagi Grimberg wrote:
On Fri, Feb 24, 2023 at 11:54:39PM +0000, Chaitanya Kulkarni wrote:
I do think that we should work on CDL for NVMe as it will solve some of
the timeout related problems effectively than using aborts or any other
mechanism.
That proposal exists in NVMe TWG, but doesn't appear to have recent
activity.
The last I heard, one point of contention was where the duration
limit property
exists: within the command, or the queue. From my perspective, if
it's not at
the queue level, the limit becomes meaningless, but hey, it's not up
to me.
Limit attached to the command makes things more flexible and easier
for the
host, so personally, I prefer that. But this has an impact on the
controller:
the device needs to pull in *all* commands to be able to know the
limits and do
scheduling/aborts appropriately. That is not something that the device
designers
like, for obvious reasons (device internal resources...).
On the other hand, limits attached to queues could lead to either a
serious
increase in the number of queues (PCI space & number of IRQ vectors
limits), or,
loss of performance as a particular queue with the desired limit would be
accessed from multiple CPUs on the host (lock contention). Tricky
problem I
think with lots of compromises.
I'm not up to speed on how CDL is defined, but I'm unclear how CDL at
the queue level would cause the host to open more queues?
Another question, does CDL have any relationship with NVMe "Time Limited
Error Recovery"? where the host can set a feature for timeout and
indicate if the controller should respect it per command?
While this is not a full-blown every queue/command has its own timeout,
it could address the original use-case given by Hannes. And it's already
there.
I guess that is the NVMe version of CDLs; can you give me a reference
for it?
Cheers,
Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
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