Re: [RFC v2 05/83] Add NOVA filesystem definitions and useful helper routines.

Eric Biggers
2018-03-19T20:30:32Z

On Mon, Mar 19, 2018 at 12:39:55PM -0700, Andiry Xu wrote:
> On Sun, Mar 11, 2018 at 12:22 PM, Eric Biggers <ebiggers3@gmail.com> wrote:
> > On Sun, Mar 11, 2018 at 02:00:13PM +0200, Nikolay Borisov wrote:
> >> [Adding Herbert Xu to CC since he is the maintainer of the crypto subsys
> >> maintainer]
> >>
> >> On 10.03.2018 20:17, Andiry Xu wrote:
> >> <snip>
> >>
> >> > +static inline u32 nova_crc32c(u32 crc, const u8 *data, size_t len)
> >> > +{
> >> > + u8 *ptr = (u8 *) data;
> >> > + u64 acc = crc; /* accumulator, crc32c value in lower 32b */
> >> > + u32 csum;
> >> > +
> >> > + /* x86 instruction crc32 is part of SSE-4.2 */
> >> > + if (static_cpu_has(X86_FEATURE_XMM4_2)) {
> >> > + /* This inline assembly implementation should be equivalent
> >> > + * to the kernel's crc32c_intel_le_hw() function used by
> >> > + * crc32c(), but this performs better on test machines.
> >> > + */
> >> > + while (len > 8) {
> >> > + asm volatile(/* 64b quad words */
> >> > + "crc32q (%1), %0"
> >> > + : "=r" (acc)
> >> > + : "r" (ptr), "0" (acc)
> >> > + );
> >> > + ptr += 8;
> >> > + len -= 8;
> >> > + }
> >> > +
> >> > + while (len > 0) {
> >> > + asm volatile(/* trailing bytes */
> >> > + "crc32b (%1), %0"
> >> > + : "=r" (acc)
> >> > + : "r" (ptr), "0" (acc)
> >> > + );
> >> > + ptr++;
> >> > + len--;
> >> > + }
> >> > +
> >> > + csum = (u32) acc;
> >> > + } else {
> >> > + /* The kernel's crc32c() function should also detect and use the
> >> > + * crc32 instruction of SSE-4.2. But calling in to this function
> >> > + * is about 3x to 5x slower than the inline assembly version on
> >> > + * some test machines.
> >>
> >> That is really odd. Did you try to characterize why this is the case? Is
> >> it purely the overhead of dispatching to the correct backend function?
> >> That's a rather big performance hit.
> >>
> >> > + */
> >> > + csum = crc32c(crc, data, len);
> >> > + }
> >> > +
> >> > + return csum;
> >> > +}
> >> > +
> >
> > Are you sure that CONFIG_CRYPTO_CRC32C_INTEL was enabled during your tests and
> > that the accelerated version was being called? Or, perhaps CRC32C_PCL_BREAKEVEN
> > (defined in arch/x86/crypto/crc32c-intel_glue.c) needs to be adjusted. Please
> > don't hack around performance problems like this; if they exist, they need to be
> > fixed for everyone.
> >
>
> I have performed the crc32c test on a Xeon X5647 at 2.93GHz, 14G DDR3
> memory at 1066MHz platform.
> You are right that enabling CONFIG_CRYPTO_CRC32C_INTEL improves the
> performance significantly. nova_crc32c() is still slightly faster than
> crc32c() with the flag enabled.
>
> Result numbers are follows: data size in bytes, latency in ns, column
> 3 is crc32c() with CONFIG_CRYPTO_CRC32C_INTEL enabled and column 4
> disabled.
>
> data size (bytes) nova_crc32c() crc32c() -enabled
> crc32c() -disabled
> 64 19 21 56
> 128 28 29 99
> 256 46 43 182
> 512 82 149 354
> 1024 157 232 728
> 2048 305 415 1440
> 4096 603 725 2869
>

Probably CRC32C_PCL_BREAKEVEN needs to be adjusted for that CPU, as I suggested
may be the case; notice that your measured speeds are about the same before 512
(CRC32C_PCL_BREAKEVEN) bytes, but the crypto API version is slower at >= 512
bytes. It would be possible to set the breakeven point in
crc32c_intel_mod_init() depending on the CPU. Again, if the performance is not
good enough you need to fix it for everyone, not hack around it.

Thanks,

Eric