netdev - Re: [PATCH net-next 0/2] net/sctp: Avoid allocating high order memory with kmalloc()

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Date:   Fri, 27 Apr 2018 01:45:13 +0300
From:   Oleg Babin <obabin@...tuozzo.com>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:     netdev@...r.kernel.org, linux-sctp@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Vlad Yasevich <vyasevich@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        Xin Long <lucien.xin@...il.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>
Subject: Re: [PATCH net-next 0/2] net/sctp: Avoid allocating high order memory
 with kmalloc()

On 04/27/2018 01:28 AM, Marcelo Ricardo Leitner wrote:
> On Fri, Apr 27, 2018 at 01:14:56AM +0300, Oleg Babin wrote:
>> Hi Marcelo,
>>
>> On 04/24/2018 12:33 AM, Marcelo Ricardo Leitner wrote:
>>> Hi,
>>>
>>> On Mon, Apr 23, 2018 at 09:41:04PM +0300, Oleg Babin wrote:
>>>> Each SCTP association can have up to 65535 input and output streams.
>>>> For each stream type an array of sctp_stream_in or sctp_stream_out
>>>> structures is allocated using kmalloc_array() function. This function
>>>> allocates physically contiguous memory regions, so this can lead
>>>> to allocation of memory regions of very high order, i.e.:
>>>>
>>>>   sizeof(struct sctp_stream_out) == 24,
>>>>   ((65535 * 24) / 4096) == 383 memory pages (4096 byte per page),
>>>>   which means 9th memory order.
>>>>
>>>> This can lead to a memory allocation failures on the systems
>>>> under a memory stress.
>>>
>>> Did you do performance tests while actually using these 65k streams
>>> and with 256 (so it gets 2 pages)?
>>>
>>> This will introduce another deref on each access to an element, but
>>> I'm not expecting any impact due to it.
>>>
>>
>> No, I didn't do such tests. Could you please tell me what methodology
>> do you usually use to measure performance properly?
>>
>> I'm trying to do measurements with iperf3 on unmodified kernel and get
>> very strange results like this:
> ...
> 
> I've been trying to fight this fluctuation for some time now but
> couldn't really fix it yet. One thing that usually helps (quite a lot)
> is increasing the socket buffer sizes and/or using smaller messages,
> so there is more cushion in the buffers.
> 
> What I have seen in my tests is that when it floats like this, is
> because socket buffers floats between 0 and full and don't get into a
> steady state. I believe this is because of socket buffer size is used
> for limiting the amount of memory used by the socket, instead of being
> the amount of payload that the buffer can hold. This causes some
> discrepancy, especially because in SCTP we don't defrag the buffer (as
> TCP does, it's the collapse operation), and the announced rwnd may
> turn up being a lie in the end, which triggers rx drops, then tx cwnd
> reduction, and so on. SCTP min_rto of 1s also doesn't help much on
> this situation.
> 
> On netperf, you may use -S 200000,200000 -s 200000,200000. That should
> help it.
>

Thank you very much! I'll try this and get back with results later.

-- 
Best regards,
Oleg