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Re: [Qemu-devel] use timer for adding latency to each block I/O

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] use timer for adding latency to each block I/O
Date: Mon, 16 May 2016 09:33:43 -0700
User-agent: Mutt/1.6.0 (2016-04-01)

On Thu, May 12, 2016 at 09:27:42AM -0500, Huaicheng Li wrote:
> My goal is to add latency for each I/O without blocking the submission path. 
> Now 
> I can know how long each I/O should wait before it’s submitted to the AIO 
> queue 
> via a model. Now the question is how can I make the I/O wait for that long 
> time
> before it’s finally handled by worker threads.

The "null" block driver supports latency simulation but the driver does
not perform real I/O.

The Linux kernel also offer latency simulation.  Take a look at the
dm-delay and scsi_debug drivers.

> I’m thinking about adding a timer for each block I/O and its callback 
> function will
> do the submission job. Below is some of my thought and questions about this 
> idea: 

The way it's done in the "null" block driver is:

static coroutine_fn int null_co_common(BlockDriverState *bs)
    BDRVNullState *s = bs->opaque;

    if (s->latency_ns) {
        co_aio_sleep_ns(bdrv_get_aio_context(bs), QEMU_CLOCK_REALTIME,
    return 0;

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