[Devel] Re: [PATCH 2.6.24-rc3-mm1] IPC: make struct ipc_ids static in ipc_namespace

[Pierre Peiffer]

Ok, I have the patch ready, but before sending it, I worry about the size of
struct ipc_namespace if we mark struct ipc_ids as ___cacheline_aligned....

Of course, you we fall into a classical match: performance vs memory size.

As I don't think that I have the knowledge to decide what we must focus on, here
after is, for info, the size reported by pahole (on x86, Intel Xeon)

With the patch sent at the beginning of this thread we have:

struct ipc_namespace {
        struct kref                kref;                 /*     0     4 */
        struct ipc_ids             ids[3];               /*     4   156 */
        /* --- cacheline 2 boundary (128 bytes) was 32 bytes ago --- */
        int                        sem_ctls[4];          /*   160    16 */
        int                        used_sems;            /*   176     4 */
        int                        msg_ctlmax;           /*   180     4 */
        int                        msg_ctlmnb;           /*   184     4 */
        int                        msg_ctlmni;           /*   188     4 */
        /* --- cacheline 3 boundary (192 bytes) --- */
        atomic_t                   msg_bytes;            /*   192     4 */
        atomic_t                   msg_hdrs;             /*   196     4 */
        size_t                     shm_ctlmax;           /*   200     4 */
        size_t                     shm_ctlall;           /*   204     4 */
        int                        shm_ctlmni;           /*   208     4 */
        int                        shm_tot;              /*   212     4 */

        /* size: 216, cachelines: 4 */
        /* last cacheline: 24 bytes */
};      /* definitions: 1 */

With the new patch, if we mark the struct ipc_ids as ____cacheline_aligned, we
have (I put kref at the end, to save one more cacheline):

struct ipc_namespace {
        struct ipc_ids             sem_ids;              /*     0    64 */

        /* XXX last struct has 12 bytes of padding */

        /* --- cacheline 1 boundary (64 bytes) --- */
        int                        sem_ctls[4];          /*    64    16 */
        int                        used_sems;            /*    80     4 */

        /* XXX 44 bytes hole, try to pack */

        /* --- cacheline 2 boundary (128 bytes) --- */
        struct ipc_ids             msg_ids;              /*   128    64 */

        /* XXX last struct has 12 bytes of padding */

        /* --- cacheline 3 boundary (192 bytes) --- */
        int                        msg_ctlmax;           /*   192     4 */
        int                        msg_ctlmnb;           /*   196     4 */
        int                        msg_ctlmni;           /*   200     4 */
        atomic_t                   msg_bytes;            /*   204     4 */
        atomic_t                   msg_hdrs;             /*   208     4 */

        /* XXX 44 bytes hole, try to pack */

        /* --- cacheline 4 boundary (256 bytes) --- */
        struct ipc_ids             shm_ids;              /*   256    64 */

        /* XXX last struct has 12 bytes of padding */

        /* --- cacheline 5 boundary (320 bytes) --- */
        size_t                     shm_ctlmax;           /*   320     4 */
        size_t                     shm_ctlall;           /*   324     4 */
        int                        shm_ctlmni;           /*   328     4 */
        int                        shm_tot;              /*   332     4 */
        struct kref                kref;                 /*   336     4 */

        /* size: 384, cachelines: 6 */
        /* sum members: 252, holes: 2, sum holes: 88 */
        /* padding: 44 */
        /* paddings: 3, sum paddings: 36 */
};      /* definitions: 1 */

We can put all sysctl related values together, in one cacheline and keep ipc_ids
cacheline aligned ? But I really wonder about the performance gain here...

Humm humm, comment ?

P.

Pavel Emelyanov wrote:
> Pierre Peiffer wrote:
>> Hi,
>>
>> 	Thanks for reviewing this !
>>
>> Pavel Emelyanov wrote:
>>> Pavel Emelyanov wrote:
>>>> Cedric Le Goater wrote:
>>>>> Pierre Peiffer wrote:
>>> [snip]
>>>
>>>>> Pavel, what do you think of it ? 
>>>> Looks sane, good catch, Pierre.
>>>>
>>>> But I'd find out whether these three ipc_ids intersect any 
>>>> cache-line. In other words I'd mark the struct ipc_ids as
>>>> ____cacheline_aligned and checked for any differences.
>>> BTW! It might be also useful to keep ipc_ids closer to their
>>> sysctl parameters.
>>>
>> It makes sense indeed.
>>
>> That would mean to have something like this, right ?
> 
> Yup :)
> 
>> struct ipc_namespace {
>> 	struct kref	kref;
>>
>> 	struct ipc_ids	sem_ids;
>> 	int		sem_ctls[4];
>> 	int		used_sems;
>>
>> 	struct ipc_ids	msg_ids;
>> 	int		msg_ctlmax;
>> 	int		msg_ctlmnb;
>> 	int		msg_ctlmni;
>> 	atomic_t	msg_bytes;
>> 	atomic_t	msg_hdrs;
>>
>> 	struct ipc_ids	shm_ids;
>> 	size_t		shm_ctlmax;
>> 	size_t		shm_ctlall;
>> 	int		shm_ctlmni;
>> 	int		shm_tot;
>> };
>>
>> After a quick look, that implies to rework a little bit procfs... othwise, it's
>> not a big deal as I can see.
> 
> Thanks!
> 
>> P.
>>
>>>>> Acked-by: Cedric Le Goater <clg at fr.ibm.com>
>>>>>
>>>>> Thanks,
>>>> Thanks,
>>>> Pavel
>>>>
>>>>> C.
>>> [snip]
>>> -
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> 
> 

-- 
Pierre Peiffer
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