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From: "Hanoch Haim (hhaim)" <hhaim@cisco.com>
To: Olivier MATZ <olivier.matz@6wind.com>,
	"bruce.richardson@intel.com" <bruce.richardson@intel.com>
Cc: "dev@dpdk.org" <dev@dpdk.org>,
	"Ido Barnea \(ibarnea\)" <ibarnea@cisco.com>,
	"Itay Marom \(imarom\)" <imarom@cisco.com>
Subject: Re: [dpdk-dev] [PATCH v2] mbuf: optimize rte_mbuf_refcnt_update
Date: Tue, 5 Jan 2016 11:11:24 +0000	[thread overview]
Message-ID: <521c3b8c6a014c24824bb1c5f17dca03@XCH-RTP-017.cisco.com> (raw)
In-Reply-To: <568BA1A1.2070300@6wind.com>

Hi Oliver, 
Thank you for the fast response and it would be great to open a discussion on that.
In general our project can leverage your optimization and I think it is great (we should have thought about it) . We can use it using the workaround I described.
However, for me it  seems odd that  rte_pktmbuf_attach () that does not *change* anything in m_const, except of the *atomic* ref counter does not work in parallel.
The example I gave is a classic use case of rte_pktmbuf_attach  (multicast ) and I don't see why it wouldn't work after your optimization. 

Do you have a pointer to the documentation that state that that you can't call the atomic ref counter from more than one thread?

Thanks,
Hanoh

-----Original Message-----
From: Olivier MATZ [mailto:olivier.matz@6wind.com] 
Sent: Tuesday, January 05, 2016 12:58 PM
To: Hanoch Haim (hhaim); bruce.richardson@intel.com
Cc: dev@dpdk.org; Ido Barnea (ibarnea); Itay Marom (imarom)
Subject: Re: [dpdk-dev] [PATCH v2] mbuf: optimize rte_mbuf_refcnt_update

Hi Hanoch,

On 01/04/2016 03:43 PM, Hanoch Haim (hhaim) wrote:
> Hi Oliver,
>
> Let's take your drawing as a reference and add my question The use 
> case is sending a duplicate multicast packet by many threads.
> I can split it to x threads to do the job and with atomic-ref (my multicast not mbuf) count it until it reaches zero.
>
> In my following example the two cores (0 and 1) sending the indirect 
> m1/m2 do alloc/attach/send
>
>      core0			             |	core1
> ---------------------------------                         |---------------------------------------
> m_const=rte_pktmbuf_alloc(mp)             |
>                                                                    |
> while true:                                                 |  while True:
>    m1 =rte_pktmbuf_alloc(mp_64)             |    m2 =rte_pktmbuf_alloc(mp_64)
>    rte_pktmbuf_attach(m1, m_const)         |    rte_pktmbuf_attach(m1, m_const)
>    tx_burst(m1)                                           |    tx_burst(m2)
>
> Is this example is not valid?

For me, m_const is not expected to be used concurrently on several cores. By "used", I mean calling a function that modifies the mbuf, which is the case for rte_pktmbuf_attach().

> BTW this is our workaround
>
>
>    core0			                    |	core1
> ---------------------------------                  |---------------------------------------
> m_const=rte_pktmbuf_alloc(mp)      |
> rte_mbuf_refcnt_update(m_const,1)| <<-- workaround
>                                                             |
> while true:                                          |  while True:
>    m1 =rte_pktmbuf_alloc(mp_64)      |    m2 =rte_pktmbuf_alloc(mp_64)
>    rte_pktmbuf_attach(m1, m_const)  |    rte_pktmbuf_attach(m1, m_const)
>    tx_burst(m1)                                     |    tx_burst(m2)

This workaround indeed solves the issue. Another solution would be to protect the call to attach() with a lock, or call all the
rte_pktmbuf_attach() on the same core.

I'm open to discuss this behavior for rte_pktmbuf_attach() function (should concurrent calls be allowed or not). In any case, we may want to better document it in the doxygen API comments.


Regards,
Olivier

  reply	other threads:[~2016-01-05 11:11 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-12-27  9:39 Hanoch Haim (hhaim)
2016-01-04 13:53 ` Olivier MATZ
2016-01-04 14:43   ` Hanoch Haim (hhaim)
2016-01-05 10:57     ` Olivier MATZ
2016-01-05 11:11       ` Hanoch Haim (hhaim) [this message]
2016-01-05 12:12         ` Olivier MATZ
2016-01-13 11:48         ` Bruce Richardson
2016-01-13 16:28           ` Hanoch Haim (hhaim)
2016-01-13 16:40             ` Bruce Richardson
  -- strict thread matches above, loose matches on Subject: below --
2015-06-01  9:32 [dpdk-dev] [PATCH] mbuf: optimize first reference increment in rte_pktmbuf_attach Olivier Matz
2015-06-08 14:57 ` [dpdk-dev] [PATCH v2] mbuf: optimize rte_mbuf_refcnt_update Olivier Matz
2015-06-09 12:57   ` Bruce Richardson
2015-06-12 14:10     ` Thomas Monjalon

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