Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH 0/1] ipc/util.{c,h}: Use binary search for max_idx
To:     Davidlohr Bueso <dave@stgolabs.net>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>, 1vier1@web.de
References: <20210425075208.11777-1-manfred@colorfullife.com>
 <20210425180729.7ew42ab5ygstbwm4@offworld>
From:   Manfred Spraul <manfred@colorfullife.com>
Message-ID: <3b503caa-2f2e-3b9a-e513-342509b668c2@colorfullife.com>
Date:   Sun, 25 Apr 2021 20:19:51 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.8.1
MIME-Version: 1.0
In-Reply-To: <20210425180729.7ew42ab5ygstbwm4@offworld>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Precedence: bulk

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