[Devel] Re: dm-ioband + bio-cgroup benchmarks

[Vivek Goyal]

On Fri, Sep 19, 2008 at 03:12:21PM +0900, Hirokazu Takahashi wrote:
> Hi,
> 
> > > Hi All,
> > > 
> > > I have got excellent results of dm-ioband, that controls the disk I/O
> > > bandwidth even when it accepts delayed write requests.
> > > 
> > > In this time, I ran some benchmarks with a high-end storage. The
> > > reason was to avoid a performance bottleneck due to mechanical factors
> > > such as seek time.
> > > 
> > > You can see the details of the benchmarks at:
> > > http://people.valinux.co.jp/~ryov/dm-ioband/hps/
> > > 
> > 
> > Hi Ryo,
> > 
> > I had a query about dm-ioband patches. IIUC, dm-ioband patches will break
> > the notion of process priority in CFQ because now dm-ioband device will
> > hold the bio and issue these to lower layers later based on which bio's
> > become ready. Hence actual bio submitting context might be different and
> > because cfq derives the io_context from current task, it will be broken.
> 
> This is completely another problem we have to solve.
> The CFQ scheduler has really bad assumption that the current process
> must be the owner. This problem occurs when you use some of device
> mapper devices or use linux aio.
> 
> > To mitigate that problem, we probably need to implement Fernando's
> > suggestion of putting io_context pointer in bio. 
> > 
> > Have you already done something to solve this issue?
> 
> Actually, I already have a patch to solve this problem, which make
> each bio have a pointer to the io_context of the owner process.
> Would you take a look at the thread whose subject is "I/O context
> inheritance" in:
> http://www.uwsg.iu.edu/hypermail/linux/kernel/0804.2/index.html#2850
> 
> Fernando also knows this.

Great. Sure I will have a look at this thread. This is something we shall
have to implement, irrespective of the fact whether we go for dm-ioband
approach or an rb-tree per request queue approach.

Thanks
Vivek
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