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Re: [dm-devel] [PATCH 9/9] dm crypt: sort writes



Hi,

This is just a comment.

I like this sorting and I would like to apply this sorting code also to dm-writeboost.
https://github.com/akiradeveloper/dm-writeboost

Hope that improves the writeback.
dm-writeboost writes back multiple (e.g. 4KB * 1000) data in cache device at once
hoping that the scheduler sorts them implicitly.
Explictly sort them possibly makes writeback more efficient.

Thanks for the code.

--
Akira

On Mon, 31 Mar 2014 08:39:40 -0400
Mike Snitzer <snitzer redhat com> wrote:

> On Sat, Mar 29 2014 at  4:11am -0400,
> Milan Broz <gmazyland gmail com> wrote:
> 
> > On 03/28/2014 09:11 PM, Mike Snitzer wrote:
> > > From: Mikulas Patocka <mpatocka redhat com>
> > > 
> > > Write requests are sorted in a red-black tree structure and are submitted
> > > in the sorted order.
> > > 
> > > In theory the sorting should be performed by the underlying disk scheduler,
> > > however, in practice the disk scheduler accepts and sorts only 128 requests.
> > > In order to sort more requests, we need to implement our own sorting.
> > 
> > Hi,
> > 
> > I think it would be nice to mention why simply increasing queue nr_request
> > doesn't help. It is definitely not limited to 128, it is only default value.
> > 
> > (I just wonder how this helps for SSDs where I think it depends what's fw
> > doing with io requests anyway).
> 
> Obviously is less of a concern on SSD but I think we'll find it still is
> beneficial.
> 
> > It would be nice to have sysfs switch to disable sorting in dmcrypt but that
> > can be added later.
> 
> Not sure sysfs is the right place to put this.  Would prefer to use a DM
> message to toggle it.  And possibly default to off if the backing
> storage is non-rotational.
> 
> > Anyway, thanks for rebasing these patches!
> > 
> > ...
> > > +#define io_node rb_entry(parent, struct dm_crypt_io, rb_node)
> > > +		if (sector < io_node->sector)
> > > +			p = &io_node->rb_node.rb_left;
> > > +		else
> > > +			p = &io_node->rb_node.rb_right;
> > > +#undef io_node
> > 
> > Btw, could this be switched to inline function instead of define,
> > or it is only me who thinks #define here is ugly? :)
> 
> I wasn't a big fan of the #define .. #undef but I didn't see the need to
> change it if Mikulas felt more comfortable with it this way.
> 
> I recently wrote some rbtree code for dm-thinp, it also uses a #define
> but outside of the function body (above the function), see:
> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=for-next&id=a43260f01a3b6f5ef33d0abc86e9b0a92096cd84
> 
> We could change this code in a similar way.
> 
> --
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