Re: [dpdk-dev] [PATCH] Implement memcmp using AVX/SSE instructio

[Ravi Kerur <rkerur@gmail.com>]

On Thu, Apr 23, 2015 at 7:00 AM, Bruce Richardson <
bruce.richardson@intel.com> wrote:

> On Thu, Apr 23, 2015 at 06:53:44AM -0700, Ravi Kerur wrote:
> > On Thu, Apr 23, 2015 at 2:23 AM, Ananyev, Konstantin <
> > konstantin.ananyev@intel.com> wrote:
> >
> > >
> > >
> > > > -----Original Message-----
> > > > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Bruce
> Richardson
> > > > Sent: Thursday, April 23, 2015 9:12 AM
> > > > To: Wodkowski, PawelX
> > > > Cc: dev@dpdk.org
> > > > Subject: Re: [dpdk-dev] [PATCH] Implement memcmp using AVX/SSE
> instructio
> > > >
> > > > On Thu, Apr 23, 2015 at 09:24:52AM +0200, Pawel Wodkowski wrote:
> > > > > On 2015-04-22 17:33, Ravi Kerur wrote:
> > > > > >+/**
> > > > > >+ * Compare bytes between two locations. The locations must not
> > > overlap.
> > > > > >+ *
> > > > > >+ * @note This is implemented as a macro, so it's address should
> not
> > > be taken
> > > > > >+ * and care is needed as parameter expressions may be evaluated
> > > multiple times.
> > > > > >+ *
> > > > > >+ * @param src_1
> > > > > >+ *   Pointer to the first source of the data.
> > > > > >+ * @param src_2
> > > > > >+ *   Pointer to the second source of the data.
> > > > > >+ * @param n
> > > > > >+ *   Number of bytes to compare.
> > > > > >+ * @return
> > > > > >+ *   true if equal otherwise false.
> > > > > >+ */
> > > > > >+static inline bool
> > > > > >+rte_memcmp(const void *src_1, const void *src,
> > > > > >+          size_t n) __attribute__((always_inline));
> > > > > You are exposing this as public API, so I think you should follow
> > > > > description bellow or not call this _memcmp_
> > > > >
> > > > > int memcmp(const void *s1, const void *s2, size_t n);
> > > > >
> > > > > The memcmp() function returns an integer less than, equal  to,  or
> > > greater
> > > > > than
> > > > >        zero  if  the  first  n  bytes  of s1 is found,
> respectively,
> > > to be
> > > > > less than, to
> > > > >        match, or be greater than the first n bytes of s2.
> > > > >
> > > >
> > > > +1 to this point.
> > > >
> > > > Also, if I read your quoted performance numbers in your earlier mail
> > > correctly,
> > > > we are only looking at a 1-4% performance increase. Is the additional
> > > code to
> > > > maintain worth the benefit?
> > >
> > > Yep, same thought here, is it really worth it?
> > > Konstantin
> > >
> > > >
> > > > /Bruce
> > > >
> > > > > --
> > > > > Pawel
> > >
> >
> > I think I haven't exploited every thing x86 has to offer to improve
> > performance. I am looking for inputs. Until we have exhausted all
> avenues I
> > don't want to drop it. One thing I have noticed is that bigger key size
> > gets better performance numbers. I plan to re-run perf tests with 64 and
> > 128 bytes key size and will report back. Any other avenues to try out
> > please let me know I will give it a shot.
> >
> > Thanks,
> > Ravi
>
> Hi Ravi,
>
> are 128 byte comparisons realistic? An IPv6 5-tuple with double vlan tags
> is still
> only 41 bytes, or 48 with some padding added?
> While for a memcpy function, you can see cases where you are going to copy
> a whole
> packet, meaning that sizes of 128B+ (up to multiple k) are realistic, it's
> harder
> to see that for a compare function.
>
> In any case, we await the results of your further optimization work to see
> how
> that goes.
>
>
Hi Bruce,

Couple of things I am planning to try

1. Use _xor_ and _testz_ instructions for comparison instead of _cmpeq_ and
_mask_.
2. I am using unaligned loads, not sure about the penalty, I plan to try
with aligned loads if address is aligned and compare results.

Agreed that with just L3 or even if we go with L2 + L3 + L4 tuples it will
not exceed 64 bytes, 128 bytes is just a stretch for some weird MPLSoGRE
header formats.

My focus is currently on improving performance for < 64 bytes and < 128
bytes key lengths only.

Thanks,
Ravi

Regards,
> /Bruce
>