Re: [PATCH][RF C/T/D] Unmapped page cache control - via boot parameter

[Avi Kivity]

On 03/16/2010 04:27 PM, Balbir Singh wrote:
> > Let's assume the guest has virtio (I agree with IDE we need
> > reordering on the host).  The guest sends batches of I/O separated
> > by cache flushes.  If the batches are smaller than the virtio queue
> > length, ideally things look like:
> >
> >   io_submit(..., batch_size_1);
> >   io_getevents(..., batch_size_1);
> >   fdatasync();
> >   io_submit(..., batch_size_2);
> >    io_getevents(..., batch_size_2);
> >    fdatasync();
> >    io_submit(..., batch_size_3);
> >    io_getevents(..., batch_size_3);
> >    fdatasync();
> >
> > (certainly that won't happen today, but it could in principle).
> >
> > How does a write cache give any advantage?  The host kernel sees
> > _exactly_ the same information as it would from a bunch of threaded
> > pwritev()s followed by fdatasync().
> >
> >      
> Are you suggesting that the model with cache=writeback gives us the
> same I/O pattern as cache=none, so there are no opportunities for
> optimization?
>    

Yes.  The guest also has a large cache with the same optimization algorithm.

>    
> > (wish: IO_CMD_ORDERED_FDATASYNC)
> >
> > If the batch size is larger than the virtio queue size, or if there
> > are no flushes at all, then yes the huge write cache gives more
> > opportunity for reordering.  But we're already talking hundreds of
> > requests here.
> >
> > Let's say the virtio queue size was unlimited.  What
> > merging/reordering opportunity are we missing on the host?  Again we
> > have exactly the same information: either the pagecache lru + radix
> > tree that identifies all dirty pages in disk order, or the block
> > queue with pending requests that contains exactly the same
> > information.
> >
> > Something is wrong.  Maybe it's my understanding, but on the other
> > hand it may be a piece of kernel code.
> >
> >      
> I assume you are talking of dedicated disk partitions and not
> individual disk images residing on the same partition.
>    

Correct. Images in files introduce new writes which can be optimized.

--
error compiling committee.c: too many arguments to function

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