linux-kernel - Re: [PATCH RFC 2/2] futex: Implement mechanism to wait on any of several futexes

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Message-ID: <306b3332-0065-59dc-e6d6-ee3c8a67ef53@gmail.com>
Date:   Wed, 31 Jul 2019 10:15:49 -0500
From:   Zebediah Figura <z.figura12@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, dvhart@...radead.org,
        linux-kernel@...r.kernel.org, kernel@...labora.com,
        Steven Noonan <steven@...vesoftware.com>,
        "Pierre-Loup A . Griffais" <pgriffais@...vesoftware.com>,
        viro@...iv.linux.org.uk, jannh@...gle.com
Subject: Re: [PATCH RFC 2/2] futex: Implement mechanism to wait on any of
 several futexes

On 7/31/19 7:06 AM, Peter Zijlstra wrote:
> On Tue, Jul 30, 2019 at 06:06:02PM -0400, Gabriel Krisman Bertazi wrote:
>> This is a new futex operation, called FUTEX_WAIT_MULTIPLE, which allows
>> a thread to wait on several futexes at the same time, and be awoken by
>> any of them.  In a sense, it implements one of the features that was
>> supported by pooling on the old FUTEX_FD interface.
>>
>> My use case for this operation lies in Wine, where we want to implement
>> a similar interface available in Windows, used mainly for event
>> handling.  The wine folks have an implementation that uses eventfd, but
>> it suffers from FD exhaustion (I was told they have application that go
>> to the order of multi-milion FDs), and higher CPU utilization.
> 
> So is multi-million the range we expect for @count ?
> 

Not in Wine's case; in fact Wine has a hard limit of 64 synchronization 
primitives that can be waited on at once (which, with the current 
user-side code, translates into 65 futexes). The exhaustion just had to 
do with the number of primitives created; some programs seem to leak 
them badly.