Re: Inexplicable I/O latency using worker threads

[Chris Snook <csnook@redhat.com>]

Thanos Makatos wrote:
> Hello all.
> 
> I am facing a weird problem with a virtual block driver I made concerning excessive I/O latency.
> 
> My block driver intercepts requests and redirects them to a real block device,
> but not just be setting the bio->bi_bdev field, I create new bios.
> 
> Anyway, my problem is that for load balancing reasons I need per-CPU worker threads
> where I enqueue requests and let them do all the work. If I use 2 threads in a round
> robin manner (request 1 served by CPU 0, 2 by CPU1, 3 by CPU0 and so on), performance
> is inexplicably low.
> 
> If I choose only one CPU to act as a worker the problem is gone. The difference of measured
> I/O latency is more than 30 times.
> 
> What could be happening?
> 
> I'm using a vanilla 2.6.18.8.
> 
> Thanx in advance.

a) I/O scheduling
b) lock contention

Do you really need to load balance I/O to a single bdev across multiple CPUs? 
Disk I/O generally isn't very CPU-intensive.

-- Chris