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From: Waiman Long <>
To: Manfred Spraul <>,
	"Luis R. Rodriguez" <>,
	Kees Cook <>,
	Andrew Morton <>,
	Jonathan Corbet <>
Cc:,,, Al Viro <>,
	Matthew Wilcox <>,
	"Eric W. Biederman" <>,
	Takashi Iwai <>, Davidlohr Bueso <>
Subject: Re: [PATCH v12 3/3] ipc: Do cyclic id allocation with ipcmni_extend mode
Date: Mon, 18 Mar 2019 14:53:11 -0400	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 03/18/2019 02:37 PM, Waiman Long wrote:
> On 03/17/2019 02:27 PM, Manfred Spraul wrote:
>> Hi Waiman,
>> On 2/28/19 7:47 PM, Waiman Long wrote:
>>> For ipcmni_extend mode, the sequence number space is only 7 bits. So
>>> the chance of id reuse is relatively high compared with the non-extended
>>> mode.
>>> To alleviate this id reuse problem, the id allocation will be done
>>> cyclically to cycle through all the 24-bit id space before wrapping
>>> around when in ipcmni_extend mode. This may cause the use of more memory
>>> in term of the number of xa_nodes allocated as well as potentially more
>>> cachelines used as the xa_nodes may be spread more sparsely in this
>>> case.
>>> There is probably a slight memory and performance cost in doing cyclic
>>> id allocation. For applications that really need more than 32k unique
>>> IPC
>>> identifiers, this is a small price to pay to avoid the id reuse problem.
>> Have you measured it?
>> I have observed -3% for semop() for a 4 level radix tree compared to a
>> 1-level radix tree, and I'm a bit reluctant to accept that.
>> Especially as the percentage will increase if the syscall overhead
>> goes down again (-> less spectre impact).
> It is both Spectre (retpoline) and Meltdown (PTI). PTI is not needed in
> AMD CPU and so you may see a bit higher slowdown.

The use of idr_replace() in your previous patch may also slow the code
path a bit to reduce the performance difference that you saw. This is
actually my main concern with using idr_replace() as suggested by
Matthew, but I am OK to use it if people think absolute correctness is
more important.


  reply	other threads:[~2019-03-18 18:53 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-28 18:47 [PATCH v12 0/3] ipc: Increase IPCMNI limit Waiman Long
2019-02-28 18:47 ` [PATCH v12 1/3] ipc: Allow boot time extension of IPCMNI from 32k to 16M Waiman Long
2019-02-28 18:47 ` [PATCH v12 2/3] ipc: Conserve sequence numbers in ipcmni_extend mode Waiman Long
2019-03-16 18:52   ` Manfred Spraul
2019-03-18 18:57     ` Waiman Long
2019-03-18 19:00     ` Waiman Long
2019-02-28 18:47 ` [PATCH v12 3/3] ipc: Do cyclic id allocation with " Waiman Long
2019-03-17 18:27   ` Manfred Spraul
2019-03-18 18:37     ` Waiman Long
2019-03-18 18:53       ` Waiman Long [this message]
     [not found]     ` <>
2019-03-19 18:18       ` Manfred Spraul
2019-03-19 18:46         ` Waiman Long

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