[dm-devel] dm-thin: Several Questions on dm-thin performance.

[Eric Wheeler]

On Fri, 6 Dec 2019, Nikos Tsironis wrote:
> On 11/22/19 8:55 PM, Joe Thornber wrote:
> > On Fri, Nov 22, 2019 at 11:14:15AM +0800, JeffleXu wrote:
> > 
> > > The first question is what's the purpose of data cell? In thin_bio_map(),
> > > normal bio will be packed as a virtual cell and data cell. I can
> > > understand
> > > that virtual cell is used to prevent discard bio and non-discard bio
> > > targeting the same block from being processed at the same time. I find it
> > > was added in commit     e8088073c9610af017fd47fddd104a2c3afb32e8 (dm thin:
> > > fix race between simultaneous io and discards to same block), but I'm
> > > still
> > > confused about the use of data cell.
> > 
> > As you are aware there are two address spaces for the locks.  The 'virtual'
> > one
> > refers to cells in the logical address space of the thin devices, and the
> > 'data' one
> > refers to the underlying data device.  There are certain conditions where we
> > unfortunately need to hold both of these (eg, to prevent a data block being
> > reprovisioned
> > before an io to it has completed).
> > 
> > > The second question is the impact of virtual cell and data cell on IO
> > > performance. If $data_block_size is large for example 1G, in multithread
> > > fio
> > > test, most bio will be buffered in cell->bios list and then be processed
> > > by
> > > worker thread asynchronously, even when there's no discard bio. Thus the
> > > original parallel IO is processed by worker thread serially now. As the
> > > number of fio test threads increase, the single worker thread can easily
> > > get
> > > CPU 100%, and thus become the bottleneck of the performance since dm-thin
> > > workqueue is ordered unbound.
> > 
> > Yep, this is a big issue.  Take a look at dm-bio-prison-v2.h, this is the
> > new interface that we need to move dm-thin across to use (dm-cache already
> > uses it).
> > It allows concurrent holders of a cell (ie, read locks), so we'll be able to
> > remap
> > much more io without handing it off to a worker thread.  Once this is done I
> > want
> > to add an extra field to cells that will cache the mapping, this way if you
> > acquire a
> > cell that is already held then you can avoid the expensive btree lookup.
> > Together
> > these changes should make a huge difference to the performance.
> > 
> > If you've got some spare coding cycles I'd love some help with this ;)
> > 
> 
> Hi Joe,
> 
> I would be interested in helping you with this task. I can't make any
> promises, but I believe I could probably spare some time to work on it.


Hi Nikos, it would be great if you are able help with the 
dm-thin port to dm-bio-prison-v2.  I'm glad to see you are interested in 
dm-thin performance too.

These are the commits that implemented dm-bio-prison-v2 in dm-cache back 
in ~4.12, maybe it can give you a good start on what the conversion might 
look like:

b29d4986d dm cache: significant rework to leverage dm-bio-prison-v2

Here's a related bugfix:

d1260e2a3 dm cache: fix race condition in the writeback mode overwrite_bio optimisation



--
Eric Wheeler


> 
> Nikos
> 
> > - Joe
> > 
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