From: Florian Weimer <fweimer@redhat.com>
To: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
Cc: DJ Delorie <dj@redhat.com>,
"libc-alpha@sourceware.org" <libc-alpha@sourceware.org>
Subject: Re: [PATCH] malloc: Use correct C11 atomics for fastbin
Date: Fri, 02 Dec 2022 12:24:14 +0100 [thread overview]
Message-ID: <87y1rqrqu9.fsf@oldenburg.str.redhat.com> (raw)
In-Reply-To: <PAWPR08MB8982C8C13946E71A8C68290B83179@PAWPR08MB8982.eurprd08.prod.outlook.com> (Wilco Dijkstra's message of "Fri, 2 Dec 2022 10:56:30 +0000")
* Wilco Dijkstra:
> Hi,
>
>>> Wilco Dijkstra via Libc-alpha <libc-alpha@sourceware.org> writes:
>>>> +/* Atomically pop from the fastbin list. The arena lock must be held to
>>>> + block other threads removing entries, avoiding the ABA issue. */
>>>
>>> If the arena lock must be held anyway, why go through the compare and
>>> exchange overhead? We know we're the only thread accessing it.
>>
>> Other threads are adding entries without the arena lock.
>
> Yes, malloc is blocking but free isn't and accesses the freelist
> concurrently. It's a really weird design. Splitting the free list
> into a local one and a shared one would be far better - no atomics
> when you have the malloc lock, and if the local free list is empty it
> takes one atomic to copy all shared entries.
The local free list is in the tcache. I think DJ said that removing the
fastbins still resulted in a performance loss.
The tcache has also the benefit that the chain length is bounded.
Thanks,
Florian
next prev parent reply other threads:[~2022-12-02 11:24 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-21 16:09 Wilco Dijkstra
2022-11-21 16:18 ` Florian Weimer
2022-11-21 16:55 ` Wilco Dijkstra
2022-11-21 16:56 ` Wilco Dijkstra
2022-11-21 17:00 ` Florian Weimer
2022-12-02 5:11 ` DJ Delorie
2022-12-02 6:36 ` Florian Weimer
2022-12-02 10:56 ` Wilco Dijkstra
2022-12-02 11:24 ` Florian Weimer [this message]
2022-12-02 12:02 ` Wilco Dijkstra
2022-12-02 18:55 ` DJ Delorie
2022-12-05 18:39 ` Zack Weinberg
2022-12-06 16:19 ` DJ Delorie
2022-12-12 3:35 ` Zack Weinberg
2022-12-12 11:57 ` Florian Weimer
2022-12-12 11:56 ` Florian Weimer
2022-12-06 13:29 ` Wilco Dijkstra
2022-12-06 13:37 ` Adhemerval Zanella Netto
2022-12-06 14:31 ` Zack Weinberg
2022-12-06 16:23 ` DJ Delorie
2022-12-15 15:43 ` Wilco Dijkstra
2022-12-02 18:55 ` DJ Delorie
2022-12-06 15:04 Wilco Dijkstra
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