linux-kernel - Re: [PATCH 0/1] ipc/util.{c,h}: Use binary search for max

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Date:   Sun, 25 Apr 2021 20:19:51 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>, 1vier1@....de
Subject: Re: [PATCH 0/1] ipc/util.{c,h}: Use binary search for max_idx

Hi Davidlohr,

On 4/25/21 8:07 PM, Davidlohr Bueso wrote:
> On Sun, 25 Apr 2021, Manfred Spraul wrote:
>
>> 2nd version of the patch:
>> @Andrew: Could you add the patch to your mm tree, as candidate for
>> linux-next?
>>
>> Note:
>> I have tried to remove the ids->max_idx cache entirely. Unfortunately,
>> this causes a significant slow-down of semstat(,,IPC_STAT):
>>   * no object allocated, no ipcmni_extended: +50%
>>   * no object allocated, with ipcmni_extended: +80%
>>   * 30 objects allocated, with large gaps, no ipcmni_extended:
>>           +350%
>> Thus I haven't removed ids->max_id.
>
> Right, IPC_STAT is the main usecase for max_id. But I'm not sure why
> you were looking to remove it in the first place - or was it just to
> avoid this patch altogether?
>
I had assumed that after removing the linear search, the lookup would be 
so fast that the cache can be removed entirely.
It would save ~20 lines of code and one int in struct ipc_ids (12 bytes 
per namespace?).

But: My assumption was wrong, the slowdown is too large.

--

     Manfred