Re: [dpdk-dev] [PATCH v2] Implement memcmp using AVX/SSE instructions.

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi Ravi,

> -----Original Message-----
> From: Ananyev, Konstantin
> Sent: Wednesday, May 13, 2015 11:02 AM
> To: Ananyev, Konstantin
> Subject: FW: [dpdk-dev] [PATCH v2] Implement memcmp using AVX/SSE instructions.
> 
> 
> 
> From: Ravi Kerur [mailto:rkerur@gmail.com]
> Sent: Monday, May 11, 2015 9:47 PM
> To: Ananyev, Konstantin
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH v2] Implement memcmp using AVX/SSE instructions.
> 
> Hi Konstantin,
> 
> On Mon, May 11, 2015 at 12:35 PM, Ananyev, Konstantin <konstantin.ananyev@intel.com> wrote:
> 
> Hi Ravi,
> 
> >
> > From: Ravi Kerur [mailto:rkerur@gmail.com]
> > Sent: Monday, May 11, 2015 6:43 PM
> > To: Ananyev, Konstantin
> > Cc: Matt Laswell; dev@dpdk.org
> > Subject: Re: [dpdk-dev] [PATCH v2] Implement memcmp using AVX/SSE instructions.
> >
> > Hi Konstantin,
> >
> >
> > On Mon, May 11, 2015 at 2:51 AM, Ananyev, Konstantin <konstantin.ananyev@intel.com> wrote:
> > Hi Ravi,
> >
> > > -----Original Message-----
> > > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ravi Kerur
> > > Sent: Friday, May 08, 2015 11:55 PM
> > > To: Matt Laswell
> > > Cc: dev@dpdk.org
> > > Subject: Re: [dpdk-dev] [PATCH v2] Implement memcmp using AVX/SSE instructions.
> > >
> > > On Fri, May 8, 2015 at 3:29 PM, Matt Laswell <laswell@infiniteio.com> wrote:
> > >
> > > >
> > > >
> > > > On Fri, May 8, 2015 at 4:19 PM, Ravi Kerur <rkerur@gmail.com> wrote:
> > > >
> > > >> This patch replaces memcmp in librte_hash with rte_memcmp which is
> > > >> implemented with AVX/SSE instructions.
> > > >>
> > > >> +static inline int
> > > >> +rte_memcmp(const void *_src_1, const void *_src_2, size_t n)
> > > >> +{
> > > >> +       const uint8_t *src_1 = (const uint8_t *)_src_1;
> > > >> +       const uint8_t *src_2 = (const uint8_t *)_src_2;
> > > >> +       int ret = 0;
> > > >> +
> > > >> +       if (n & 0x80)
> > > >> +               return rte_cmp128(src_1, src_2);
> > > >> +
> > > >> +       if (n & 0x40)
> > > >> +               return rte_cmp64(src_1, src_2);
> > > >> +
> > > >> +       if (n & 0x20) {
> > > >> +               ret = rte_cmp32(src_1, src_2);
> > > >> +               n -= 0x20;
> > > >> +               src_1 += 0x20;
> > > >> +               src_2 += 0x20;
> > > >> +       }
> > > >>
> > > >>
> > > > Pardon me for butting in, but this seems incorrect for the first two cases
> > > > listed above, as the function as written will only compare the first 128 or
> > > > 64 bytes of each source and return the result.  The pattern expressed in
> > > > the 32 byte case appears more correct, as it compares the first 32 bytes
> > > > and then lets later pieces of the function handle the smaller remaining
> > > > bits of the sources. Also, if this function is to handle arbitrarily large
> > > > source data, the 128 byte case needs to be in a loop.
> > > >
> > > > What am I missing?
> > > >
> > >
> > > Current max hash key length supported is 64 bytes, hence no comparison is
> > > done after 64 bytes. 128 bytes comparison is added to measure performance
> > > only and there is no use-case as of now. With the current use-cases its not
> > > required but if there is a need to handle large arbitrary data upto 128
> > > bytes it can be modified.
> > So on x86 let say rte_memcmp(k1, k2, 65) might produce invalid results, right?
> > While on PPC will work as expected (as it calls memcpu underneath)?
> > That looks really weird to me.
> > If you plan to use rte_memcmp only for hash comparisons, then probably
> > you should put it somewhere into librte_hash and name it accordingly: rte_hash_key_cmp() or something.
> > And put a big comment around it, that it only works with particular lengths.
> > If you want it to be a generic function inside EAL, then it probably need to handle different lengths properly
> > on all supported architectures.
> > Konstantin
> >
> >
> > Let me just explain it here and probably add it to document as well.
> >
> > rte_memcmp is not
> >
> > 1. a replacement to memcmp
> >
> > 2.  restricted to hash key comparison
> >
> > rte_memcmp is
> >
> > 1. optimized comparison for 16 to 128 bytes, v1 patch series had this support. Changed some of the logic in v2 due to concerns raised
> > for unavailable use-cases beyond 64 bytes comparison.
> From what I see in v2 it supposed to work correctly for len in [0,64] and  len=128, right?
> Not sure I get it: so for v1 it was able to handle any length correctly, but then you removed it?
> If so, I wonder what was the reason? Make it faster?
> 
> My initial discussion was with Zhilong(John) from Intel and we decided to implement up to 128 bytes comparison and use rte_hash
> and rte_lpm6 as a candidate for testing. When I sent out v1 patch, Bruce comments were on use-case for 128 bytes comparison and
> was it really required? Hence I decided in v2 to support only up to 64 bytes and added 128 bytes only for performance measurement.
> Personally I think support for up to 128 bytes comparison is required, there might not be use-cases today but it will definitely be
> useful.

Ok, we don't have a real usage case for it now, but it still probably good to have it work with arbitrary key-length.
Again, as Don suggested in another mail, we can have an optimised implementation
for particular sizes and fall back to slow-path (memcp) for all other cases.
Even if you'll decide to limit len to particular value (64/128), it is probably not very good to have a gap in between,
as it exists now [65-127].

> 
> Another thing that looks strange to me:
> While all rte_cmp*() uses actual data values for comparison results,
> rte_memcmp_remainder() return value depends not only on data values but also on data locations:
> 
> +static inline int
> +rte_memcmp_remainder(const uint8_t *src_1u, const uint8_t *src_2u, size_t n)
> +{
> ...
> exit:
> +
> +       return src_1u < src_2u ? -1 : 1;
> +}
> 
> This is a bug and its not supposed to be there. I will fix it. Thanks for catching it.
> 
> If you just test for equal/not equal that doesn't really matter.
> If this is supposed to be a 'proper' comparison function, then the result is sort of unpredictable.
> > With minor tuning over the weekend I am able to get better performance for
> > anything between 16 to 128 bytes comparison.
> >
> > 2. will be specific to DPDK  i.e. currently all memcmp usage in DPDK are for equality or inequality hence "less than" or "greater than"
> > implementation in rte_memcmp doesn't make sense and will be removed in subsequent patches, it will return 0 or 1 for
> > equal/unequal cases.
> 
> If you don't plan your function to follow memcmp() semantics and syntax, why to name it rte_memcmp()?
> I  think that will make a lot of confusion around.
> Why not to name it differently(and put a clear comment in the declaration of course)?
> 
> Following memcmp semantics is not hard but there are no use-cases for it in DPDK currently. Keeping it specific to DPDK usage
> simplifies code as well. I can change the name to "rte_compare" and add comments to the function. Will it work?

Yep, either rte_compare(), or as Don suggested rte_testequal() - both seems good to me.

Konstantin

> 
> 
> >
> > rte_hash will be the first candidate to move to rte_memcmp and subsequently rte_lpm6 which uses 16 bytes comparison will be
> > moved
> >
> > Later on RING_SIZE which uses large size for comparison will be moved. I am currently studying/understanding that logic and will
> make
> > changes to rte_memcmp to support that.
> 
> Sorry, didn't get you here.
> 
> Once rte_hash, rte_lpm6 changes and new compare function code are reviewed and accepted I plan to move to different
> components (RING_SIZE is currently defined to be from 256 to 16384 bytes) and memcmp function being used in test_ring,
> test_pmd_ring and other functions. I did not want to add all component changes into one patch series as it causes high review latency
> or patch series just dies down silently. Instead make patches small and incremental in every series, hope this clarifies.
> Thanks,
> Ravi
> Konstantin
> 
> >
> > I don't want to make lot of changes in one shot and see that patch series die a slow death with no takers.
> >
> > Thanks,
> > Ravi
> >
> > >
> > > >
> > > > --
> > > > Matt Laswell
> > > > infinite io, inc.
> > > > laswell@infiniteio.com
> > > >
> > > >