It depends on what you're trying to accomplish. I see 2 advantages of
using mmap:
1. the data is persistent. If you want the data persistent across boot,
mmap-ed data is in a file.
When you start your processes after a reboot, with mmap the data is
already there. You just mmap the file(s) and you're done. (Of course,
you may not want/need this).
2. mmap is easier to debug. If you need to look at the data being
shared, you can simply cat/od the file. With shmget/shmat, you have to
write a program (or use a debugger).
However, shmget/shmat will be marginally faster (at least in initial
access), as they don't go through file system code. Once the data is in
memory, there should be no difference. (I guess this also depends on
the size of the memory you are mapping,and the amount of memory on the
system). I haven't looked
at perf diffs with SHM_SHARE_MMU, so maybe that also makes a
difference. As far as I know, there is no flag you can use with mmap to
get the page table(s) themselves shared.
max
Borse, Ganesh wrote:
Thanks for time.
I did not think of using mmap. (Rather I'm bit unsure about what to use when -
so I statyed with shmat).
Yes, I when I gave address instead of (void*)0 with shmat, it attached to same
address.
What would you prefer? - mmap with address or shmat with adress? - or both are
same?
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
Sent: 04 April 2007 17:39
To: Borse, Ganesh
Cc: 'perf-discuss@opensolaris.org'
Subject: Re: [perf-discuss] Request for guidance on shmat() call attaching at
different addre ss
Hi Ganesh,
I have done the same kind of thing you are trying to do with shmget/shmat, but
I used mmap. With mmap, I needed to specify the address (first arg to
mmap(2)) with
the MAP_FIXED flag. Have you tried specifying an address instead of the
"(void*)0"?
(I admit, using SHM_SHARE_MMU flag seems like it would end up giving the same
address, but that is only an assumption).
max
Borse, Ganesh wrote:
No, it is not the address (the 2nd param) passed to shmat calls. I'm passing
"(void*)0".
It is the address returned by shmat, after attaching the segment.
My code is as below:-
int id = shmget(key,size,(0777 | IPC_CREAT | IPC_EXCL)); if(errno ==
EEXIST){
id = shmget(key,size,(0777 | IPC_CREAT)); }
suht->_dataMemSegments[idx]._dataMemory = (pSUHT_ITEM)shmat(id, (void
suht->*)0, 0777 | SHM_SHARE_MMU);
if(NULL == suht->_dataMemSegments[idx]._dataMemory || errno != 0){
printf("shm:_dataMemory:failed: errno:%d\n",errno);
return errno;
}
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
Sent: 04 April 2007 16:49
To: Borse, Ganesh
Cc: 'perf-discuss@opensolaris.org'
Subject: Re: [perf-discuss] Request for guidance on shmat() call
attaching at different addre ss
Hi Genesh,
Are you specifying an address in your shmat calls? Can you post code segments doing shmget/shmat?
max
Borse, Ganesh wrote:
Hi,
I am facing a problem of shared memory segment getting attached at
different address, when the processes using it go down and attach
count goes to zero.
This causes the problem when the processes next time come up & try to
access the addresses taken from the shared memory segment.
The scenario in brief:-
I've 3 processes using multiple shared memory segments. Shared memory
segments (shm) are also used in hierarchical fashion.
One shm is used as a high level data structure to hold pointers to
other shms.
Second shm is an array of pointers to structures (data blocks) which
will get allocated in the third shm. For the other 2 processes to
access these blocks, their addresses will be stored in this second
shm (an array).
The third shm actually holds the data as retrieved by the server
process from database. This is acting as cache.
When the third shm gets completely used, the server process creates
new shm and starts storing data into it. Once all data gets stored,
server process goes down - without removing any shm.
Next when the client processes comes up, they attach to all these
shms created by server process.
When the client process calls "shmat" on the first 3 memory segments,
shmat returns the same address which it returned to server process
(these addresses are stored in 1st shm).
But, when client process tries to attach to 4th segment (which was
created after the third data shm filled up), the shmat returns
totally different address than that of returned to the server process.
Since, the addresses stored in array shm correspond to the different
shm address, when the client process tries to access those addresses,
it crashes with SIGSEGV.
I am using SHM_SHARE_MMU flag with shmat to use the feature of ISM &
to get same address.
Any clue/ hint why only the 4th (last segment) shmat returns
different address? Is this the normal behavior?
Could I be doing something wrong only while creating this segment?
Please help.
Thanks and Regards,
GaneshB
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