Re: dm-writecache

On Fri, 18 May 2018, Dan Williams wrote: > >> ...and I wonder what the benefit is of the 16-byte case? I would > >> assume the bulk of the benefit is limited to the 4 and 8 byte copy > >> cases. > > > > dm-writecache uses 16-byte writes frequently, so it is needed for that. > > > > If we split 16-byte write to two 8-byte writes, it would degrade > > performance for architectures where memcpy_flushcache needs to flush the > > cache. > > My question was how measurable it is to special case 16-byte > transfers? I know Ingo is going to ask this question, so it would > speed things along if this patch included performance benefit numbers > for each special case in the changelog. I tested it some times ago - and the movnti instruction has 2% better throughput than the existing memcpy_flushcache function. It is doing one 16-byte write for every sector written and one 8-byte write for every sector clean-up. So, the overhead is measurable.