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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