Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling

Ross Zwisler
2015-11-16T19:48:54Z

On Mon, Nov 16, 2015 at 09:28:59AM -0800, Dan Williams wrote:
> On Mon, Nov 16, 2015 at 6:05 AM, Jan Kara <jack@suse.cz> wrote:
> > On Mon 16-11-15 14:37:14, Jan Kara wrote:
> [..]
> > But a question: Won't it be better to do sfence + pcommit only in response
> > to REQ_FLUSH request and don't do it after each write? I'm not sure how
> > expensive these instructions are but in theory it could be a performance
> > win, couldn't it? For filesystems this is enough wrt persistency
> > guarantees...
>
> We would need to gather the performance data... The expectation is
> that the cache flushing is more expensive than the sfence + pcommit.

I think we should revisit the idea of removing wmb_pmem() from the I/O path in
both the PMEM driver and in DAX, and just relying on the REQ_FUA/REQ_FLUSH
path to do wmb_pmem() for all cases. This was brought up in the thread
dealing with the "big hammer" fsync/msync patches as well.

https://lkml.org/lkml/2015/11/3/730

I think we can all agree from the start that wmb_pmem() will have a nonzero
cost, both because of the PCOMMIT and because of the ordering caused by the
sfence. If it's possible to avoid doing it on each I/O, I think that would be
a win.

So, here would be our new flows:

PMEM I/O:
write I/O(s) to the driver
PMEM I/O writes the data using non-temporal stores

REQ_FUA/REQ_FLUSH to the PMEM driver
wmb_pmem() to order all previous writes and flushes, and to
PCOMMIT the dirty data durably to the DIMMs

DAX I/O:
write I/O(s) to the DAX layer
write the data using regular stores (eventually to be replaced
with non-temporal stores)

flush the data with wb_cache_pmem() (removed when we use
non-temporal stores)

REQ_FUA/REQ_FLUSH to the PMEM driver
wmb_pmem() to order all previous writes and flushes, and to
PCOMMIT the dirty data durably to the DIMMs

DAX msync/fsync:
writes happen to DAX mmaps from userspace

DAX fsync/msync
all dirty pages are written back using wb_cache_pmem()

REQ_FUA/REQ_FLUSH to the PMEM driver
wmb_pmem() to order all previous writes and flushes, and to
PCOMMIT the dirty data durably to the DIMMs

DAX/PMEM zeroing (suggested by Dave: https://lkml.org/lkml/2015/11/2/772):
PMEM driver receives zeroing request
writes a bunch of zeroes using non-temporal stores

REQ_FUA/REQ_FLUSH to the PMEM driver
wmb_pmem() to order all previous writes and flushes, and to
PCOMMIT the dirty data durably to the DIMMs

Having all these flows wait to do wmb_pmem() in the PMEM driver in response to
REQ_FUA/REQ_FLUSH has several advantages:

1) The work done and guarantees provided after each step closely match the
normal block I/O to disk case. This means that the existing algorithms used
by filesystems to make sure that their metadata is ordered properly and synced
at a known time should all work the same.

2) By delaying wmb_pmem() until REQ_FUA/REQ_FLUSH time we can potentially do
many I/Os at different levels, and order them all with a single wmb_pmem().
This should result in a performance win.

Is there any reason why this wouldn't work or wouldn't be a good idea?