Hi Michael,
Yes I added the "MAP_NOINIT" to mmap() and "UNMAP_INIT_OPTIONAL" flag to
munmap_flags() call. Don't know where i might be going wrong in SQLite.
As you suggested, I wrote a small application to check if this works.
Fortunately it worked as desired (as given below).
MAP_NOINIT
When specified, the POSIX requirement that the memory be zeroed is relaxed. The
physical memory being used for this allocation must have been previously freed
with UNMAP_INIT_OPTIONAL for this flag to have any effect.
_________________________________________________________________________________
int main(int argc, char *argv[]) {
printf("Welcome to the QNX Momentics IDE\n");
//Open a file
int fd = open("/tmp/mmaptest",O_RDWR|O_CREAT, 0777);
//Truncate the file
int size = ftruncate(fd, 2048);
const char buff[]="Testing the mmap API";
write(fd,buff,sizeof(buff));
//Mapping the disk file to memory
void *pMem = mmap(0, 2048, PROT_READ|PROT_WRITE,MAP_SHARED, fd, 0);
//Check the mmapped memory contents
printf("Memory Mapped Data: %s\n", pMem);
//Set all mmapped locations to 'A'
memset(pMem,'A',2048);
//Unmap the memory
//munmap(pMem, 2048);
munmap_flags(pMem, 2048, UNMAP_INIT_OPTIONAL);
//Buffer to read disk data
char disk_read[2048];
//Read the data from the file to check if its synced
read(fd,disk_read,sizeof(disk_read));
printf("Reading disk file data:%s\n",disk_read);
//Resetting the contents of disk file
size = ftruncate(fd, 0);
close(fd);
//Open the file again to check to re-map the memory
fd = open("/tmp/mmaptest",O_RDWR, 0777);
size = ftruncate(fd, 2048);
//Map the memory region again
//int pMem1 = mmap(0, 2048, PROT_READ|PROT_WRITE,MAP_SHARED, fd, 0);
int pMem1 = mmap(0, 2048, PROT_READ|PROT_WRITE,MAP_SHARED|MAP_NOINIT, fd,
0);
//Print the re-mapped region
printf("New Memory Mapped Data: %s\n", pMem1);
//Read the disk contents again to check if "MAP_NOINIT" works
read(fd,disk_read,sizeof(disk_read));
printf("Disk file data during re-map: %s\n", disk_read);
//Close the disk file
close(fd);
return EXIT_SUCCESS;
}
---------------------------------------------------------------
Output:
Welcome to the QNX Momentics IDE
Memory Mapped Data: Testing the mmap API
Reading disk file data:AAAAAAAAAAAAAAAAAAAAAA [2048 times]
New Memory Mapped Data: AAAAAAAAAAAAAAAAAAAAA [2048 times]
Disk file data during re-map:
---------------------------------------------------------------
Also found that the mmapped data is written back to disk file on
munmap()/munmap_flags() call.
Maybe i need to look into Sqlite amalgamation file in detail to analyze what's
happening here during unmapping of memory.
I believe we can use these flags under all scenarios without any
pre-conditions. Not sure if POSIX (on QNX) have some limitations on their usage.
Thanks,
Praveen
______________________________________________________________________________________________________________________
-----Original Message-----
From: [email protected] [mailto:[email protected]]
On Behalf Of Black, Michael (IS)
Sent: Thursday, October 14, 2010 9:22 PM
To: General Discussion of SQLite Database
Subject: Re: [sqlite] TestFixture 3.7.2 - Some WAL tests fail on QNX OS
Did you also add the MAP_NOINIT to the mmap() call?
It sounds like exactly the behavior you're seeing.
Did you try writing a stand-alone app to test this idea?
Michael D. Black
Senior Scientist
Advanced Analytics Directorate
Northrop Grumman Information Systems
________________________________
From: [email protected] on behalf of Raj, Praveen
Sent: Thu 10/14/2010 10:44 AM
To: General Discussion of SQLite Database
Subject: EXTERNAL:Re: [sqlite] TestFixture 3.7.2 - Some WAL tests fail on QNX OS
Hi Micheal,
Thanks Dan and Michael for all your inputs.
I tried this approach as well, but didn't find any success.
During unmapping i used the API munmap_flags() with "UNMAP_INIT_OPTIONAL" flag
to avoid the zero initialization during the next mmaping.
Another thought I have here is that the old mmapped regions may not be
initialized with zeros, but instead the regions are getting synced with the
data in disk file (which is full of zeros). Not sure if can happen with mmap()
API though?
Thanks,
Praveen
-----Original Message-----
From: [email protected] [mailto:[email protected]]
On Behalf Of Black, Michael (IS)
Sent: Thursday, October 14, 2010 5:21 PM
To: General Discussion of SQLite Database
Subject: Re: [sqlite] TestFixture 3.7.2 - Some WAL tests fail on QNX OS
I sent this before...have you tried this?
According to the QNX mmap page
http://www.qnx.com/developers/docs/6.3.0SP3/neutrino/lib_ref/m/mmap.html
<https://owa1.ngc.com/exchweb/bin/redir.asp?URL=http://www.qnx.com/developers/docs/6.3.0SP3/neutrino/lib_ref/m/mmap.html>
MAP_NOINIT
When specified, the POSIX requirement that the memory be zeroed is relaxed. The
physical memory being used for this allocation must have been previously freed
with UNMAP_INIT_OPTIONAL for this flag to have any effect.
________________________________
This flag was added in the QNX Neutrino Core OS 6.3.2.
Interesting that this claims it's a POSIX requirement but I don't think most
any others do this.
Michael D. Black
Senior Scientist
Advanced Analytics Directorate
Northrop Grumman Information Systems
________________________________
From: [email protected] on behalf of Raj, Praveen
Sent: Thu 10/14/2010 5:53 AM
To: General Discussion of SQLite Database
Subject: EXTERNAL:Re: [sqlite] TestFixture 3.7.2 - Some WAL tests fail on QNX OS
Hi Dan,
I did some more investigation on the issue and i feel there is synchronization
problem happening here.
After mmapping the shm (wal index) file to process memory, the WAL indexes are
written into the mmapped area, and this data is not getting synchronized with
physical (shm) file. As a result when the mmap() function is called the second
time to map the 32k-64k memory region, it is synchronizing the complete mmapped
region (previous 32k regions) with physical file, even though a valid offset is
passed. Not sure if this is the actual behaviour of mmap() call.
While debugging, before the mmap() call i checked mmapped region and it had
valid indexes, whereas after the call all became 0's. Also i found that the shm
file is always filled with 0's even after commits.
When i added the msync() statement (to sync the shm file) before mmap call as
shown below, the problem is not seen. In this case the shm file has valid
32-bit indexes, as data is synchronized before next mmap call is executed.
while(pShmNode->nRegion<=iRegion){
int ret = msync( apNew[0], iRegion*szRegion, MS_SYNC);
void *pMem = mmap(0, szRegion, PROT_READ|PROT_WRITE,
MAP_SHARED, pShmNode->h, pShmNode->nRegion*szRegion
);
With the above msync() call all my failed test cases are passing.
I don't see any msync() call in the SQLite amalgamation/wal.c file.
I believe the data in mapped region and physical file are not synched
automatically. We need to explicitly do it using msync() call.
Don't know if there is any other mechanism in SQLite through which the data is
synchronized. Does the call to sqlite3OsSync() sync the shm file as well? or is
the shm file not syned purposefully?
This is all my understanding and not sure if this is causing the actual issue.
Please guide me if my approach/understanding is incorrect.
Thanks,
Praveen
-----Original Message-----
From: [email protected] [mailto:[email protected]]
On Behalf Of Dan Kennedy
Sent: Friday, October 08, 2010 9:33 PM
To: General Discussion of SQLite Database
Subject: Re: [sqlite] TestFixture 3.7.2 - Some WAL tests fail on QNX OS
On Oct 8, 2010, at 9:44 PM, Raj, Praveen wrote:
> Hello,
>
> I debugged the SQLite functions and here is my finding:
>
> The call to "mmap" in the function "unixShmMap" is causing the issue.
> void *pMem = mmap(0, szRegion, PROT_READ|PROT_WRITE,
> MAP_SHARED, pShmNode->h, iRegion*szRegion);
>
> It is setting the previous memory region/regions to zero while mapping
> the new ones. Mmap call internally uses the QNX API mmap64() to map
> the required memory region. Not sure on what is happening here. Just
> need to dig into memory mapping to find whats happening and hopefully
> find a solution.
>
> Dan - Do you have any idea on why this could be happening?
Sounds like a bug in QNX to me.
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