Re: Inexplicable I/O latency using worker threads

["Thanos McAtos" <mcatos@ics.forth.gr>]

> 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
What does I/O scheduling has to do with from where the I/O request
originates?
> b) lock contention
I doubt there is lock contention, my simple test uses 1 outstanding I/O.
>
> Do you really need to load balance I/O to a single bdev across multiple
> CPUs?
Yes, because I have to do very CPU-intensive operations (compression etc.).
> Disk I/O generally isn't very CPU-intensive.
>
> -- Chris
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