From: Ferruh Yigit <ferruh.yigit@amd.com>
To: Long Li <longli@microsoft.com>, stephen <stephen@networkplumber.org>
Cc: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>,
"dev@dpdk.org" <dev@dpdk.org>
Subject: Re: [Patch v2] net/mana: use rte_pktmbuf_alloc_bulk for allocating RX WQEs
Date: Thu, 1 Feb 2024 10:52:14 +0000 [thread overview]
Message-ID: <8de3d7e0-6a16-48d2-8cf2-f129441e8a1b@amd.com> (raw)
In-Reply-To: <MN0PR21MB32640C60E08795B35881DE9BCE432@MN0PR21MB3264.namprd21.prod.outlook.com>
On 2/1/2024 3:55 AM, Long Li wrote:
>>>> 'mbufs' is temporarily storage for allocated mbuf pointers, why not
>>>> allocate if from stack instead, can be faster and easier to manage:
>>>> "struct rte_mbuf *mbufs[count]"
>>>
>>> That would introduce a variable length array.
>>> VLA's should be removed, they are not supported on Windows and many
>>> security tools flag them. The problem is that it makes the code
>>> brittle if count gets huge.
>>>
>>> But certainly regular calloc() or alloca() would work here.
>>>
>>
>> Most of the existing bulk alloc already uses VLA but I can see the problem it is not
>> being supported by Windows.
>>
>> As this mbuf pointer array is short lived within the function, and being in the fast
>> path, I think continuous alloc and free can be prevented,
>>
>> one option can be to define a fixed size, big enough, array which requires
>> additional loop for the cases 'count' size is bigger than array size,
>>
>> or an array can be allocated by driver init in device specific data ,as we know it
>> will be required continuously in the datapath, and it can be freed during device
>> close()/uninit().
>>
>> I think an fixed size array from stack is easier and can be preferred.
>
> I sent a v3 of the patch, still using alloc().
>
> I found two problems with using a fixed array:
> 1. the array size needs to be determined in advance. I don't know what a good number should be. If too big, some of them may be wasted. (and maybe make a bigger mess of CPU cache) If too small, it ends up doing multiple allocations, which is the problem this patch trying to solve.
>
I think default burst size 32 can be used like below:
struct rte_mbuf *mbufs[32];
loop: //use do {} while(); if you prefer
n = min(32, count);
rte_pktmbuf_alloc_bulk(mbufs, n);
for (i = 0; i < n; i++)
mana_post_rx_wqe(rxq, mbufs[i]);
count -= n;
if (count > 0) goto loop:
This additional loop doesn't make code very complex (I think not more
than additional alloc() & free()) and it doesn't waste memory.
I suggest doing a performance measurement with above change, it may
increase performance,
afterwards if you insist to go with original code, we can do it.
> 2. if makes the code slightly more complex ,but I think 1 is the main problem.
>
> I think another approach is to use VLA by default, but for Windows use alloc().
>
> Long
next prev parent reply other threads:[~2024-02-01 10:52 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-25 2:42 [PATCH] " longli
2024-01-26 0:29 ` Stephen Hemminger
2024-01-26 1:13 ` Long Li
2024-01-30 1:13 ` [Patch v2] " longli
2024-01-30 10:19 ` Ferruh Yigit
2024-01-30 16:43 ` Stephen Hemminger
2024-01-30 18:05 ` Tyler Retzlaff
2024-01-30 22:42 ` Ferruh Yigit
2024-02-01 3:55 ` Long Li
2024-02-01 10:52 ` Ferruh Yigit [this message]
2024-02-02 1:21 ` Long Li
2024-02-01 16:33 ` Tyler Retzlaff
2024-02-02 1:22 ` Long Li
2024-01-30 21:30 ` Long Li
2024-01-30 22:34 ` Ferruh Yigit
2024-01-30 22:36 ` Long Li
2024-02-01 3:45 ` [Patch v3] " longli
2024-02-01 16:16 ` Tyler Retzlaff
2024-02-01 19:41 ` Long Li
2024-02-02 1:19 ` [Patch v4] net/mana: use rte_pktmbuf_alloc_bulk for allocating RX mbufs longli
2024-02-02 16:24 ` Stephen Hemminger
2024-02-06 18:06 ` Ferruh Yigit
2024-02-07 4:50 ` Long Li
2024-02-09 0:02 ` [Patch v5] net/mana: use rte_pktmbuf_alloc_bulk for allocating RX WQEs longli
2024-02-09 17:46 ` Ferruh Yigit
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