RE: [External] Re: [PATCH] dm writecache: skip writecache_wait for pmem mode
From: Huaisheng HS1 Ye
Date: Wed Sep 04 2019 - 11:18:56 EST
> -----Original Message-----
> From: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> Sent: Wednesday, September 4, 2019 4:49 PM
> On Mon, 2 Sep 2019, Huaisheng Ye wrote:
> > From: Huaisheng Ye <yehs1@xxxxxxxxxx>
> > The array bio_in_progress only have chance to be increased and
> > decreased with ssd mode. For pmem mode, they are not involved at all.
> > So skip writecache_wait_for_ios in writecache_flush for pmem.
> > Suggested-by: Doris Yu <tyu1@xxxxxxxxxx>
> > Signed-off-by: Huaisheng Ye <yehs1@xxxxxxxxxx>
> > ---
> > drivers/md/dm-writecache.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> > diff --git a/drivers/md/dm-writecache.c b/drivers/md/dm-writecache.c
> > index c481947..d06b8aa 100644
> > --- a/drivers/md/dm-writecache.c
> > +++ b/drivers/md/dm-writecache.c
> > @@ -726,7 +726,8 @@ static void writecache_flush(struct dm_writecache *wc)
> > }
> > writecache_commit_flushed(wc);
> > - writecache_wait_for_ios(wc, WRITE);
> > + if (!WC_MODE_PMEM(wc))
> > + writecache_wait_for_ios(wc, WRITE);
> > wc->seq_count++;
> > pmem_assign(sb(wc)->seq_count, cpu_to_le64(wc->seq_count));
> > --
> > 126.96.36.199
> I think this is not needed - wait_event in writecache_wait_for_ios exits
> immediatelly if the condition is true.
> This code path is not so hot that we would need microoptimizations like this to
> avoid function calls.
Thanks for your reply, I see what you mean, but I can't agree with you.
For pmem mode, this code path (writecache_flush) is much more hot than SSD mode.
Because in the code, the AUTOCOMMIT_BLOCKS_PMEM has been defined to 64, which means if more than 64 blocks have been inserted to cache device, also called uncommitted, writecache_flush would be called.
Otherwise, there is a timer callback function will be called every 1000 milliseconds.
#define AUTOCOMMIT_BLOCKS_SSD 65536
#define AUTOCOMMIT_BLOCKS_PMEM 64
#define AUTOCOMMIT_MSEC 1000
So when dm-writecache running in working mode, there are continuous WRITE operations has been mapped to writecache_map, writecache_flush will be used much more often than SSD mode.