On Wed, May 30 2018 at 10:46P -0400, Mikulas Patocka <mpatocka@xxxxxxxxxx> 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 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
