[RFC PATCHv4 2/6] nvme-multipath: add support for adaptive I/O policy

[Hannes Reinecke]

On 11/4/25 11:45, Nilay Shroff wrote:
> This commit introduces a new I/O policy named "adaptive". Users can
> configure it by writing "adaptive" to "/sys/class/nvme-subsystem/nvme-
> subsystemX/iopolicy"
> 
> The adaptive policy dynamically distributes I/O based on measured
> completion latency. The main idea is to calculate latency for each path,
> derive a weight, and then proportionally forward I/O according to those
> weights.
> 
> To ensure scalability, path latency is measured per-CPU. Each CPU
> maintains its own statistics, and I/O forwarding uses these per-CPU
> values. Every ~15 seconds, a simple average latency of per-CPU batched
> samples are computed and fed into an Exponentially Weighted Moving
> Average (EWMA):
> 
> avg_latency = div_u64(batch, batch_count);
> new_ewma_latency = (prev_ewma_latency * (WEIGHT-1) + avg_latency)/WEIGHT
> 
> With WEIGHT = 8, this assigns 7/8 (~87.5%) weight to the previous
> latency value and 1/8 (~12.5%) to the most recent latency. This
> smoothing reduces jitter, adapts quickly to changing conditions,
> avoids storing historical samples, and works well for both low and
> high I/O rates. Path weights are then derived from the smoothed (EWMA)
> latency as follows (example with two paths A and B):
> 
>      path_A_score = NSEC_PER_SEC / path_A_ewma_latency
>      path_B_score = NSEC_PER_SEC / path_B_ewma_latency
>      total_score  = path_A_score + path_B_score
> 
>      path_A_weight = (path_A_score * 100) / total_score
>      path_B_weight = (path_B_score * 100) / total_score
> 
> where:
>    - path_X_ewma_latency is the smoothed latency of a path in nanoseconds
>    - NSEC_PER_SEC is used as a scaling factor since valid latencies
>      are < 1 second
>    - weights are normalized to a 0–64 scale across all paths.
> 
> Path credits are refilled based on this weight, with one credit
> consumed per I/O. When all credits are consumed, the credits are
> refilled again based on the current weight. This ensures that I/O is
> distributed across paths proportionally to their calculated weight.
> 
> Signed-off-by: Nilay Shroff <nilay at linux.ibm.com>
> ---
>   drivers/nvme/host/core.c      |  15 +-
>   drivers/nvme/host/ioctl.c     |  31 ++-
>   drivers/nvme/host/multipath.c | 425 ++++++++++++++++++++++++++++++++--
>   drivers/nvme/host/nvme.h      |  74 +++++-
>   drivers/nvme/host/pr.c        |   6 +-
>   drivers/nvme/host/sysfs.c     |   2 +-
>   6 files changed, 530 insertions(+), 23 deletions(-)
> 
Reviewed-by: Hannes Reinecke <hare at suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare at suse.de                                +49 911 74053 688
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