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From: Olivier Matz <olivier.matz@6wind.com>
To: Alexander Kozyrev <akozyrev@mellanox.com>
Cc: dev@dpdk.org, viacheslavo@mellanox.com, matan@mellanox.com,
	thomas@monjalon.net, stable@dpdk.org
Subject: Re: [dpdk-dev] [PATCH v2] mbuf: optimize memory loads during mbuf freeing
Date: Fri, 27 Mar 2020 09:13:14 +0100	[thread overview]
Message-ID: <20200327081314.GD6327@platinum> (raw)
In-Reply-To: <1584719715-17818-1-git-send-email-akozyrev@mellanox.com>

Hi,

On Fri, Mar 20, 2020 at 03:55:15PM +0000, Alexander Kozyrev wrote:
> Introduction of pinned external buffers doubled memory loads in the
> rte_pktmbuf_prefree_seg() function. Analysis of the generated assembly
> code shows unnecessary load of the pool field of the rte_mbuf structure.
> Here is the snippet of the assembly for "if (!RTE_MBUF_DIRECT(m))":
> Before the change the code was:
> 	movq  0x18(%rbx), %rax // load the ol_flags field
> 	test %r13, %rax	       // check if ol_flags equals to 0x60...0
> 	jz 0x9a8718 <Block 2>  // jump out to "if (m->next != NULL)"
> After the change the code became:
> 	movq  0x18(%rbx), %rax // load ol_flags
> 	test %r14, %rax	       // check if ol_flags equals to 0x60...0
> 	jnz 0x9bea38 <Block 2> // jump in to "if (!RTE_MBUF_HAS_EXTBUF(m)"
> 	movq  0x48(%rbx), %rax // load the pool field
> 	jmp 0x9bea78 <Block 7> // jump out to "if (m->next != NULL)"
> Look like this absolutely unneeded memory load of the pool field is an
> optimization for the external buffer case in GCC (4.8.5), since Clang
> generates the same assembly for both before and after the change versions.
> Plus, GCC favors the external buffer case over the simple case.
> This assembly code layout causes the performance degradation because the
> rte_pktmbuf_prefree_seg() function is a part of a very hot path.
> Workaround this compilation issue by moving the check for pinned buffer
> apart from the check for external buffer and restore the initial code
> flow that favors the direct mbuf case over the external one.
> 
> Fixes: 6ef1107ad4c6 ("mbuf: detach mbuf with pinned external buffer")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Alexander Kozyrev <akozyrev@mellanox.com>
> Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>

Acked-by: Olivier Matz <olivier.matz@6wind.com>

Thanks!

  reply	other threads:[~2020-03-27  8:13 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-16 18:31 [dpdk-dev] [PATCH] " Alexander Kozyrev
2020-03-19  9:30 ` Olivier Matz
2020-03-20 15:35   ` Alexander Kozyrev
2020-03-20 15:55 ` [dpdk-dev] [PATCH v2] " Alexander Kozyrev
2020-03-27  8:13   ` Olivier Matz [this message]
2020-03-31  1:46     ` [dpdk-dev] [dpdk-stable] " Thomas Monjalon

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