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

[Mike Snitzer]

On Sat, Mar 29 2014 at  4:11am -0400,
Milan Broz <gmazyland at gmail.com> wrote:

> On 03/28/2014 09:11 PM, Mike Snitzer wrote:
> > From: Mikulas Patocka <mpatocka at 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.