I have found that when my process has a lot of threads each of which opens a
DIFFERENT database, they each block on eachother while opening each database.
Here is a little rust program to demonstrate the problem: <https://
pastebin.com/QGXFp6w6> If you simply remove the line "PRAGMA journal_mode =
WAL;", then the program runs more than 5x faster.
This is at least on conflict with the documentation, as the documentation
suggests that a thread is a process for the purposes of sqlite's concurrency,
at least when OPEN_NO_MUTEX is specified. I also feel like it's a bug in
general, as opening two unrelated databases should not block eachother.
Each thread blocks at this point:
#0 __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
#1 0x00007ffff74e4b95 in __GI___pthread_mutex_lock (mutex=0x7ffff7dd8148
<staticMutexes.10049+360>)
at ../nptl/pthread_mutex_lock.c:80
#2 0x00007ffff7b26419 in unixEnterMutex () at sqlite3.c:31952
#3 unixLock (id=0x7ffff4a25180, eFileLock=1) at sqlite3.c:32894
#4 0x00007ffff7b1f2fa in sqlite3OsLock (lockType=1, id=<optimized out>) at
sqlite3.c:21299
#5 pagerLockDb (pPager=pPager@entry=0x7ffff4a25008, eLock=eLock@entry=1) at
sqlite3.c:50293
#6 0x00007ffff7b1f34b in pagerLockDb (eLock=1, pPager=0x7ffff4a25008) at
sqlite3.c:53054
#7 pager_wait_on_lock (pPager=pPager@entry=0x7ffff4a25008,
locktype=locktype@entry=1) at sqlite3.c:53051
#8 0x00007ffff7b55d6c in sqlite3PagerSharedLock (pPager=0x7ffff4a25008) at
sqlite3.c:54293
#9 0x00007ffff7b56835 in lockBtree (pBt=0x7ffff4a151e8) at sqlite3.c:64591
#10 sqlite3BtreeBeginTrans (p=0x7ffff4a1b508, wrflag=wrflag@entry=0) at
sqlite3.c:64956
#11 0x00007ffff7b82997 in sqlite3InitOne (db=0x7ffff4a12008, iDb=iDb@entry=0,
pzErrMsg=pzErrMsg@entry=0x7ffff51fe778) at sqlite3.c:119558
#12 0x00007ffff7b82aca in sqlite3Init (db=0x7ffff4a12008,
pzErrMsg=pzErrMsg@entry=0x7ffff51fe778)
at sqlite3.c:119740
#13 0x00007ffff7b82b00 in sqlite3ReadSchema
(pParse=pParse@entry=0x7ffff51fe770) at sqlite3.c:119765
#14 0x00007ffff7b8d8e4 in sqlite3Pragma (pParse=0x7ffff51fe770,
pId1=pId1@entry=0x7ffff51fddd0,
pId2=pId2@entry=0x7ffff51fdde8, pValue=pValue@entry=0x7ffff51fde18,
minusFlag=minusFlag@entry=0)
at sqlite3.c:117300
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