Re: [PATCH -mm 1/3] sysv ipc: increase msgmnb default value wrt. the number of cpus

Nadia Derbey
2008-06-25T10:34:46Z

Andrew Morton wrote:
> On Tue, 24 Jun 2008 11:34:53 +0200
> <Solofo.Ramangalahy@bull.net> wrote:
>
>
>>From: Solofo Ramangalahy <Solofo.Ramangalahy@bull.net>
>>
>>Initialize msgmnb value to
>>min(MSGMNB * num_online_cpus(), MSGMNB * MSG_CPU_SCALE)
>>to increase the default value for larger machines.
>>
>>MSG_CPU_SCALE scaling factor is defined to be 4, as 16384 x 4 = 65536
>>is an already used and recommended value.
>>
>>The msgmni value is made dependant of msgmnb to keep the memory
>>dedicated to message queues within the 1/MSG_MEM_SCALE of lowmem
>>bound.
>>
>>Unlike msgmni, the value is not scaled (down) with respect to the
>>number of ipc namespaces for simplicity.
>>
>>To disable recomputation when user explicitely set a value,
>>we reuse the callback defined for msgmni.
>>
>>As msgmni and msgmnb are correlated, user settings of any of the two
>>disable recomputation of both, for now. This is refined in a later
>>patch.
>>
>>When a negative value is put in /proc/sys/kernel/msgmnb
>>automatic recomputing is re-enabled.
>>
>
>
> Thanks for taking the time to describe this work so well.
>
>
>>---
>> Documentation/sysctl/kernel.txt | 28 ++++++++++++++++++++++++++++
>> include/linux/msg.h | 6 ++++++
>> ipc/ipc_sysctl.c | 5 +++--
>> ipc/msg.c | 17 +++++++++++++----
>> 4 files changed, 50 insertions(+), 6 deletions(-)
>>
>>Index: b/ipc/msg.c
>>===================================================================
>>--- a/ipc/msg.c
>>+++ b/ipc/msg.c
>>@@ -38,6 +38,7 @@
>> #include <linux/rwsem.h>
>> #include <linux/nsproxy.h>
>> #include <linux/ipc_namespace.h>
>>+#include <linux/cpumask.h>
>>
>> #include <asm/current.h>
>> #include <asm/uaccess.h>
>>@@ -92,7 +93,7 @@ void recompute_msgmni(struct ipc_namespa
>>
>> si_meminfo(&i);
>> allowed = (((i.totalram - i.totalhigh) / MSG_MEM_SCALE) * i.mem_unit)
>>- / MSGMNB;
>>+ / ns->msg_ctlmnb;
>> nb_ns = atomic_read(&nr_ipc_ns);
>> allowed /= nb_ns;
>>
>>@@ -108,11 +109,19 @@ void recompute_msgmni(struct ipc_namespa
>>
>> ns->msg_ctlmni = allowed;
>> }
>>+/*
>>+ * Scale msgmnb with the number of online cpus, up to 4x MSGMNB.
>>+ */
>>+void recompute_msgmnb(struct ipc_namespace *ns)
>>+{
>>+ ns->msg_ctlmnb =
>>+ min(MSGMNB * num_online_cpus(), MSGMNB * MSG_CPU_SCALE);
>>+}
>>
>> void msg_init_ns(struct ipc_namespace *ns)
>> {
>> ns->msg_ctlmax = MSGMAX;
>>- ns->msg_ctlmnb = MSGMNB;
>>+ recompute_msgmnb(ns);
>>
>> recompute_msgmni(ns);
>>
>>@@ -132,8 +141,8 @@ void __init msg_init(void)
>> {
>> msg_init_ns(&init_ipc_ns);
>>
>>- printk(KERN_INFO "msgmni has been set to %d\n",
>>- init_ipc_ns.msg_ctlmni);
>>+ printk(KERN_INFO "msgmni has been set to %d, msgmnb to %d\n",
>>+ init_ipc_ns.msg_ctlmni, init_ipc_ns.msg_ctlmnb);
>>
>> ipc_init_proc_interface("sysvipc/msg",
>> " key msqid perms cbytes qnum lspid lrpid uid gid cuid cgid stime rtime ctime\n",
>>Index: b/include/linux/msg.h
>>===================================================================
>>--- a/include/linux/msg.h
>>+++ b/include/linux/msg.h
>>@@ -58,6 +58,12 @@ struct msginfo {
>> * more than 16 GB : msgmni = 32K (IPCMNI)
>> */
>> #define MSG_MEM_SCALE 32
>>+/*
>>+ * Scaling factor to compute msgmnb: ns->msg_ctlmnb is between MSGMNB
>>+ * and MSGMNB * MSG_CPU_SCALE. This leads to a max msgmnb value of
>>+ * 65536 which is an already used and recommended value.
>>+ */
>>+#define MSG_CPU_SCALE 4
>>
>> #define MSGMNI 16 /* <= IPCMNI */ /* max # of msg queue identifiers */
>> #define MSGMAX 8192 /* <= INT_MAX */ /* max size of message (bytes) */
>>Index: b/ipc/ipc_sysctl.c
>>===================================================================
>>--- a/ipc/ipc_sysctl.c
>>+++ b/ipc/ipc_sysctl.c
>>@@ -42,6 +42,7 @@ static void tunable_set_callback(int val
>> * Re-enable automatic recomputing only if not already
>> * enabled.
>> */
>>+ recompute_msgmnb(current->nsproxy->ipc_ns);
>> recompute_msgmni(current->nsproxy->ipc_ns);
>> cond_register_ipcns_notifier(current->nsproxy->ipc_ns);
>> }
>>@@ -210,8 +211,8 @@ static struct ctl_table ipc_kern_table[]
>> .data = &init_ipc_ns.msg_ctlmnb,
>> .maxlen = sizeof (init_ipc_ns.msg_ctlmnb),
>> .mode = 0644,
>>- .proc_handler = proc_ipc_dointvec,
>>- .strategy = sysctl_ipc_data,
>>+ .proc_handler = proc_ipc_callback_dointvec,
>>+ .strategy = sysctl_ipc_registered_data,
>> },
>> {
>> .ctl_name = KERN_SEM,
>>Index: b/Documentation/sysctl/kernel.txt
>>===================================================================
>>--- a/Documentation/sysctl/kernel.txt
>>+++ b/Documentation/sysctl/kernel.txt
>>@@ -179,6 +179,34 @@ kernel stack.
>>
>> ==============================================================
>>
>>+msgmnb
>>+
>>+Maximum size in bytes (not in message count) of a single SystemV IPC
>>+message queue (b stands for bytes).
>>+
>>+This value is dynamic and depends on the online cpu count of the
>>+machine (taking cpu hotplug into account).
>>+
>>+Computed values are between MSGMNB and MSGMNB*MSG_CPU_SCALE #define
>>+constants (currently [16384,65536]).
>>+
>>+The exact value is automatically (re)computed, but:
>>+. If the value is positioned from user space (via procfs or sysctl()),
>>+ to a positive value then the automatic recomputation is
>>+ disabled. This leaves control to user space. E.g.
>>+
>>+ # echo 16384 > /proc/sys/kernel/msgmnb
>>+
>>+. If the value is positioned from user space to a negative value, then
>>+ the computation is reenabled. E.g.
>>+
>>+ # echo -1 > /proc/sys/kernel/msgmnb
>>+
>>+See recompute_msgmnb() function in ipc/ directory for details.
>>+The value of msgmnb is coupled with the value of msgmni.
>>+
>
>
> The magical positive-versus-negative number trick is a bit obscure, and
> I don't think there's any precedent for it in the kernel ABI (which is
> what this is).
>
> Is there anything we can do to reduce the unusualness of this
> interface? Say, add a new /proc/sys/kernel/automatic-msgmnb which
> contains the automatic scaling and leave /proc/sys/kernel/msgmnb
> containing the manual scaling? Or something like that?

Well, I plead guilty ;-)
I've done this proposal when sending the msgmni scaling patches
(unfortunatly my network is down, so can't look the reference thread).
From what I have in my folders here's the complete story:

. January 08: sent the patches
. 02/05/2008: got an answer from Yasunori Goto:

Yasunori Goto wrote:
> Hmmm. I suppose this may be side effect which user does not wish.
>
> I would like to recommend there should be a switch which can turn
> on/off
> automatic recomputing.
> If user would like to change this value, it should be turned off.
> Otherwise, his requrest will be rejected with some messages.
>
> Probably, user can understand easier than this side effect.

. 02/11/2008: resent the patches after fixing the issues:

Nadia Derbey wrote:
> Resending the set of patches after Yasunori's remark about being able
> to turn on/off automatic recomputing.
> (see message at http://lkml.org/lkml/2008/2/5/149).
> I actually introduced an intermediate solution: when msgmni is set by
> hand, it is uneregistered from the ipcns notifier chain (i.e.
> automatic recomputing is disabled). This corresponds to an implicit
> turn off. Setting it to a negative value makes it registered back in
> the notifier chain (which corresponds to the turn on proposed by
> Yasunaori).

And I don't remember anybody complaining about that :-(

Sorry for introducing this "magical positive-vs-negative # trick".

Will think a bit more about your suggestion.

Regards,
Nadia