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

[Mike Snitzer]

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