[dm-devel] [patch 4/4] dm-writecache: use new API for flushing

[Mikulas Patocka]

On Wed, 30 May 2018, Mike Snitzer wrote:

> On Wed, May 30 2018 at 10:46P -0400,
> Mikulas Patocka <mpatocka at redhat.com> wrote:
> 
> > 
> > 
> > On Wed, 30 May 2018, Mike Snitzer wrote:
> > 
> > > > > Fine I'll deal with it.  reordering the fields eliminated holes in the
> > > > > structure and reduced struct members spanning cache lines.
> > > > 
> > > > And what about this?
> > > > #define WC_MODE_PMEM(wc)                        ((wc)->pmem_mode)
> > > > 
> > > > The code that I had just allowed the compiler to optimize out 
> > > > persistent-memory code if we have DM_WRITECACHE_ONLY_SSD defined - and you 
> > > > deleted it.
> > > > 
> > > > Most architectures don't have persistent memory and the dm-writecache 
> > > > driver could work in ssd-only mode on them. On these architectures, I 
> > > > define
> > > > #define WC_MODE_PMEM(wc)                        false
> > > > - and the compiler will just automatically remove the tests for that 
> > > > condition and the unused branch. It does also eliminate unused static 
> > > > functions.
> > > 
> > > This level of microoptimization can be backfilled.  But as it was, there
> > > were too many #defines.  And I'm really not concerned with eliminating
> > > unused static functions for this case.
> > 
> > I don't see why "too many defines" would be a problem.
> > 
> > If I compile it with and without pmem support, the difference is 
> > 15kB-vs-12kB. If we look at just one function (writecache_map), the 
> > difference is 1595 bytes - vs - 1280 bytes. So, it produces real savings 
> > in code size.
> > 
> > The problem with performance is not caused a condition that always jumps 
> > the same way (that is predicted by the CPU and it causes no delays in the 
> > pipeline) - the problem is that a bigger function consumes more i-cache. 
> > There is no reason to include code that can't be executed.
> 
> Please double check you see the reduced code size you're expecting using
> the latest dm-writecache.c in linux-dm.git's dm-4.18 branch.
> 
> Thanks,
> Mike

I checked that - it's OK.

Mikulas