Re: Document POSIX MQ /proc/sys/fs/mqueue files
From: Michael Kerrisk (man-pages)
Date: Tue Sep 30 2014 - 06:12:51 EST
On Mon, Sep 29, 2014 at 7:28 PM, Doug Ledford <dledford@xxxxxxxxxx> wrote:
> On Mon, 2014-09-29 at 11:10 +0200, Michael Kerrisk (man-pages) wrote:
>> Hello Doug, David,
>> I think you two were the last ones to make significant
>> changes to the semantics of the files in /proc/sys/fs/mqueue,
>> so I wonder if you (or anyone else who is willing) might
>> take a look at the man page text below that I've written
>> (for the mq_overview(7) page) to describe past and current
>> reality, and let me know of improvements of corrections.
>> By the way, Doug, your commit ce2d52cc1364 appears to have
>> changed/broken the semantics of the files in the /dev/mqueue
>> filesystem. Formerly, the QSIZE field in these files showed
>> the number of bytes of real user data in all of the queued
>> messages. After that commit, QSIZE now includes kernel
>> overhead bytes, which does not seem very useful for user
>> space. Was that change intentional? I see no mention of the
>> change in the commit message, so it sounds like it was not
> That change didn't come in that commit. That commit modified it, but
> didn't introduce it.
(Which commit was it then? d6629859b36 ?)
> Now, was it intentional? Yes. Is it valuable, useful? That depends on
> your perspective.
Thanks for the detailed explanation below. However, I don't understand
why the (useful) work that you describe below necessitated a change in
the QSIZE value that was exposed to user space. Surely the necessary
changes could have been done internally while still leaving QSIZE to
expose the same value it ever did? As things stand now (and unless I
am missing something), QSIZE exposes an implementation-specific
internal value that has little meaning or value to user space. And,
it's unfortunate that the commit message made no mention of the fact
that there was an ABI change here.
> The man page below looks fine to me.
Thanks for checking it!
> It covers the various
> incarnations. If I add some tweaks to the priorities value though, it
> will need updating again ;-)
> Although this section wasn't included below, I would update how the
> memory is calculated to match what I wrote above. However, I would also
> put in a notation that the calculation can change when the kernel's
> internal implementation changes and resource usage therefore changes.
>> From mq_overview(7) draft:
>> /proc interfaces
>> The following interfaces can be used to limit the amount of kerâ
>> nel memory consumed by POSIX message queues and to set the
>> default attributes for new message queues:
>> /proc/sys/fs/mqueue/msg_default (since Linux 3.5)
>> This file defines the value used for a new queue's
>> mq_maxmsg setting when the queue is created with a call to
>> mq_open(3) where attr is specified as NULL. The default
>> value for this file is 10. The minimum and maximum are as
>> for /proc/sys/fs/mqueue/msg_max. If msg_default exceeds
>> msg_max, a new queue's default mq_maxmsg value is capped
>> to the msg_max limit. Up until Linux 2.6.28, the default
>> mq_maxmsg was 10; from Linux 2.6.28 to Linux 3.4, the
>> default was the value defined for the msg_max limit.
>> This file can be used to view and change the ceiling value
>> for the maximum number of messages in a queue. This value
>> acts as a ceiling on the attr->mq_maxmsg argument given to
>> mq_open(3). The default value for msg_max is 10. The
>> minimum value is 1 (10 in kernels before 2.6.28). The
>> upper limit is HARD_MSGMAX. The msg_max limit is ignored
>> for privileged processes (CAP_SYS_RESOURCE), but the
>> HARD_MSGMAX ceiling is nevertheless imposed.
>> The definition of HARD_MSGMAX has changed across kernel
>> * Up to Linux 2.6.32: 131072 / sizeof(void *)
>> * Linux 2.6.33 to 3.4: (32768 * sizeof(void *) / 4)
>> * Since Linux 3.5: 65,536
>> /proc/sys/fs/mqueue/msgsize_default (since Linux 3.5)
>> This file defines the value used for a new queue's mq_msgâ
>> size setting when the queue is created with a call to
>> mq_open(3) where attr is specified as NULL. The default
>> value for this file is 8192. The minimum and maximum are
>> as for /proc/sys/fs/mqueue/msgsize_max. If msgâ
>> size_default exceeds msgsize_max, a new queue's default
>> mq_msgsize value is capped to the msgsize_max limit. Up
>> until Linux 2.6.28, the default mq_msgsize was 8192; from
>> Linux 2.6.28 to Linux 3.4, the default was the value
>> defined for the msgsize_max limit.
>> This file can be used to view and change the ceiling on
>> the maximum message size. This value acts as a ceiling on
>> the attr->mq_msgsize argument given to mq_open(3). The
>> default value for msgsize_max is 8192 bytes. The minimum
>> value is 128 (8192 in kernels before 2.6.28). The upper
>> limit for msgsize_max has varied across kernel versions:
>> * Before Linux 2.6.28, the upper limit is INT_MAX.
>> * From Linux 2.6.28 to 3.4, the limit is 1,048,576.
>> * Since Linux 3.5, the limit is 16,777,216 (HARD_MSGSIZEâ
>> The msgsize_max limit is ignored for privileged process
>> (CAP_SYS_RESOURCE), but, since Linux 3.5, the HARD_MSGâ
>> SIZEMAX ceiling is enforced for privileged processes.
>> This file can be used to view and change the system-wide
>> limit on the number of message queues that can be created.
>> The default value for queues_max is 256. The semantics of
>> this limit have changed across kernel versions as follows:
>> * Before Linux 3.5, this limit could be changed to any
>> value in the range 0 to INT_MAX, but privileged proâ
>> cesses (CAP_SYS_RESOURCE) can exceed the limit.
>> * Since Linux 3.5, there is a ceiling for this limit of
>> 1024 (HARD_QUEUESMAX). Privileged processes
>> (CAP_SYS_RESOURCE) can exceed the queues_max limit, but
>> the HARD_QUEUESMAX limit is enforced even for priviâ
>> leged processes.
>> * Starting with Linux 3.14, the HARD_QUEUESMAX ceiling is
>> removed: no ceiling is imposed on the queues_max limit,
>> and privileged processes (CAP_SYS_RESOURCE) can exceed
>> the limit.
> Doug Ledford <dledford@xxxxxxxxxx>
> GPG KeyID: 0E572FDD
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/