On Wed, Jun 16, 2021 at 10:24 AM David Laight <David.Laight@xxxxxxxxxx> wrote: > > From: Matteo Croce > > Sent: 16 June 2021 03:02 > ... > > > > That's a good idea, but if you read the replies to Gary's original > > > > patch > > > > https://lore.kernel.org/linux-riscv/20210216225555.4976-1-gary@xxxxxxxxxxx/ > > > > .. both Gary, Palmer and David would rather like a C-based version. > > > > This is one attempt at providing that. > > > > > > Yep, I prefer C as well :) > > > > > > But if you check commit 04091d6, the assembly version was introduced > > > for KASAN. So if we are to change it back to C, please make sure KASAN > > > is not broken. > > > > ... > > Leaving out the first memcpy/set of every test which is always slower, (maybe > > because of a cache miss?), the current implementation copies 260 Mb/s when > > the low order bits match, and 114 otherwise. > > Memset is stable at 278 Mb/s. > > > > Gary's implementation is much faster, copies still 260 Mb/s when euqlly placed, > > and 230 Mb/s otherwise. Memset is the same as the current one. > > Any idea what the attainable performance is for the cpu you are using? > Since both memset and memcpy are running at much the same speed > I suspect it is all limited by the writes. > > 272MB/s is only 34M writes/sec. > This seems horribly slow for a modern cpu. > So is this actually really limited by the cache writes to physical memory? > > You might want to do some tests (userspace is fine) where you > check much smaller lengths that definitely sit within the data cache. > I get similar results in userspace, this tool write to RAM with variable data width: root@beaglev:~/src# ./unalign_check 1 0 1 size: 1 Mb write size: 8 bit unalignment: 0 byte elapsed time: 0.01 sec throughput: 124.36 Mb/s # ./unalign_check 1 0 8 size: 1 Mb write size: 64 bit unalignment: 0 byte elapsed time: 0.00 sec throughput: 252.12 Mb/s > It is also worth checking how much overhead there is for > short copies - they are almost certainly more common than > you might expect. > This is one problem with excessive loop unrolling - the 'special > cases' for the ends of the buffer start having a big effect > on small copies. > I too believe that they are much more common than long ones. Indeed, I wish to reduce the MIN_THRESHOLD value from 64 to 32 or even 16. Or having it dependend on the word size, e.g. sizeof(long) * 2. Suggestions? > For cpu that support misaligned memory accesses, one 'trick' > for transfers longer than a 'word' is to do a (probably) misaligned > transfer of the last word of the buffer first followed by the > transfer of the rest of the buffer (overlapping a few bytes at the end). > This saves on conditionals and temporary values. > > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) > Regards, -- per aspera ad upstream
