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

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.