performance: memcpy vs. __copy_tofrom_user

[Grant Likely]

Forwarding message to linuxppc-dev at ozlabs.org.  This is an interesting
question for the wider powerpc community, but not many people read
linuxppc-embedded.

On Wed, Oct 08, 2008 at 04:39:13PM +0200, Dominik Bozek wrote:
> Hi all,
> 
> I have done a test of memcpy() and __copy_tofrom_user() on the mpc8313.
> And the major conclusion is that __copy_tofrom_user is more efficient
> than memcpy. Sometimes about 40%.
> 
> If I good understand, the memcpy() just copy the data, while
> __copy_tofrom_user() take care if the memory wasn't swapped out. So then
> memcpy() shall be faster than __copy_tofrom_user(). Am I right?
> Is here anybody, who can confirm such results and maybe is able to
> improve the memcpy()?
> 
> 
> Let talk about the test.
> I have prepared two pieces of memory of size 64KB and I make sure that
> this memory is not swapped out (necessary for memcpy() later). Then I
> run one of the memory copy function to transfer 32MB and I measure the
> time. The memory is copied in chunks from 64KB to 8B. I take care about
> the cache calling flush_dcache_range() whenever whole 64KB was used.
> I know, that memcpy on the kernel level is not intended to copy memory
> blocks in userspace and __copy_tofrom_user is not intended to copy data
> only between two user blocks, but for the performance test it doesn't
> matter.
> Bellow you may see the short piece of code in the kernel module.
> 
> #define TEST_BUF_SIZE (64*1024)
> int function;
> char *buf1, *buf2, *buf1_bis, *buf2_bis;
> unsigned int size, cnt;
> 
> get_user(function, &((TEST_ARG*)(arg))->function);
> get_user(buf1, &((TEST_ARG*)(arg))->buf1);
> get_user(buf2, &((TEST_ARG*)(arg))->buf2);
> get_user(size, &((TEST_ARG*)(arg))->size);
> 
> cnt = (32*1024*1024)/size; /* how many repeats of memory copy is needed
> to transfer 32MB ? */
> buf1_bis = buf1;
> buf2_bis = buf2;
> 
> switch (function)
> {
>     case MEMCPY_TEST:
>         while (cnt-->0)
>         {
>             if (buf1_bis >= buf1+TEST_BUF_SIZE)
>             {
>                 /* need for flusch data cache as seldom as possible */
>                 buf1_bis = buf1;
>                 buf2_bis = buf2;
>                 flush_dcache_range((int)buf1, (int)(buf2+TEST_BUF_SIZE));
>             }
>             if (buf1_bis != memcpy(buf1_bis, buf2_bis, size))
>                 break;
>             buf1_bis += size;
>             buf2_bis += size;
>         }
>         break;
> 
>     case COPY_TOFROM_USER_TEST:
>         while (cnt-->0)
>         {
>             if (buf1_bis >= buf1+TEST_BUF_SIZE)
>             {
>                 /* need for flusch data cache as seldom as possible */
>                 buf1_bis = buf1;
>                 buf2_bis = buf2;
>                 flush_dcache_range((int)buf1, (int)(buf2+TEST_BUF_SIZE));
>             }
>             ret = __copy_tofrom_user(buf1_bis, buf2_bis, size);
>             if (ret != 0)
>                 break;
>             buf1_bis += size;
>             buf2_bis += size;
>         }
>         break;
> }
> 
> 
> Bellow are the results:
> 
> memcpy()
> chunk:  65536 [B] | transfer:     69.2 [MB/s] | time: 1.849727 [s] |
> size:  128.000 [MB]
> chunk:  32768 [B] | transfer:     69.2 [MB/s] | time: 1.849700 [s] |
> size:  128.000 [MB]
> chunk:  16384 [B] | transfer:     69.2 [MB/s] | time: 1.849845 [s] |
> size:  128.000 [MB]
> chunk:   8192 [B] | transfer:     69.2 [MB/s] | time: 1.850535 [s] |
> size:  128.000 [MB]
> chunk:   4096 [B] | transfer:     69.1 [MB/s] | time: 1.853405 [s] |
> size:  128.000 [MB]
> chunk:   2048 [B] | transfer:     69.1 [MB/s] | time: 1.852877 [s] |
> size:  128.000 [MB]
> chunk:   1024 [B] | transfer:     69.2 [MB/s] | time: 1.849963 [s] |
> size:  128.000 [MB]
> chunk:    512 [B] | transfer:     69.0 [MB/s] | time: 1.853793 [s] |
> size:  128.000 [MB]
> chunk:    256 [B] | transfer:     68.6 [MB/s] | time: 1.866222 [s] |
> size:  128.000 [MB]
> chunk:    128 [B] | transfer:     68.0 [MB/s] | time: 1.883002 [s] |
> size:  128.000 [MB]
> chunk:     64 [B] | transfer:     67.2 [MB/s] | time: 1.904073 [s] |
> size:  128.000 [MB]
> chunk:     32 [B] | transfer:     64.7 [MB/s] | time: 1.978109 [s] |
> size:  128.000 [MB]
> chunk:     16 [B] | transfer:     54.5 [MB/s] | time: 2.348682 [s] |
> size:  128.000 [MB]
> chunk:      8 [B] | transfer:     47.4 [MB/s] | time: 2.698635 [s] |
> size:  128.000 [MB]
> 
> 
> __copy_tofrom_user()
> chunk:  65536 [B] | transfer:     97.3 [MB/s] | time: 1.315155 [s] |
> size:  128.000 [MB]
> chunk:  32768 [B] | transfer:     97.3 [MB/s] | time: 1.315762 [s] |
> size:  128.000 [MB]
> chunk:  16384 [B] | transfer:     97.2 [MB/s] | time: 1.316946 [s] |
> size:  128.000 [MB]
> chunk:   8192 [B] | transfer:     96.8 [MB/s] | time: 1.321705 [s] |
> size:  128.000 [MB]
> chunk:   4096 [B] | transfer:     96.6 [MB/s] | time: 1.325516 [s] |
> size:  128.000 [MB]
> chunk:   2048 [B] | transfer:     96.6 [MB/s] | time: 1.325570 [s] |
> size:  128.000 [MB]
> chunk:   1024 [B] | transfer:     96.8 [MB/s] | time: 1.322599 [s] |
> size:  128.000 [MB]
> chunk:    512 [B] | transfer:     97.8 [MB/s] | time: 1.308186 [s] |
> size:  128.000 [MB]
> chunk:    256 [B] | transfer:    100.2 [MB/s] | time: 1.277788 [s] |
> size:  128.000 [MB]
> chunk:    128 [B] | transfer:     91.5 [MB/s] | time: 1.398216 [s] |
> size:  128.000 [MB]
> chunk:     64 [B] | transfer:     87.0 [MB/s] | time: 1.471784 [s] |
> size:  128.000 [MB]
> chunk:     32 [B] | transfer:     75.0 [MB/s] | time: 1.706426 [s] |
> size:  128.000 [MB]
> chunk:     16 [B] | transfer:     47.8 [MB/s] | time: 2.678039 [s] |
> size:  128.000 [MB]
> chunk:      8 [B] | transfer:     41.5 [MB/s] | time: 3.084689 [s] |
> size:  128.000 [MB]
> 
> Regards
> Dominik Bozek
> 
> 
> BTW. The memcpy() maybe optimized as it is on i32 when the size of block
> is known at compile time.
> 
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