> I’ve just discovered the thread in the original app decreases the
> available memory by around 4 GB. Are they really that expensive?
After others pointed out threads weren’t that expensive I concluded there was a
bug in my code. On checking though I couldn’t find anything wrong yet the
programme was seemingly making 4 GB disappear as it ran. It gets even stranger
though and I’ve written the console app below to illustrate. It prints out the
amount of RAM available every 20 million steps. All other front end apps were
closed during running although there were background tasks running. It was run
in 64 bit mode although it also works for 32 bit.
#include <vcl.h>
#include <windows.h>
#pragma hdrstop
#pragma argsused
#include <tchar.h>
#include <stdio.h>
#include <conio.h>
#include <iostream>
#include "sqlite.h"
uint64_t FreeMBs()
{
MEMORYSTATUSEX status;
status.dwLength = sizeof(status);
GlobalMemoryStatusEx(&status);
return status.ullAvailPhys / (1024 * 1024);
}
int _tmain(int argc, _TCHAR* argv[])
{
const int Million=1000000;
const int Gap=20*Million;
sqlite3 *DB;
sqlite3_open("c:/SQLiteData/MyTemp.db",&DB);
sqlite3_stmt *asc,*desc;
sqlite3_prepare_v2(DB,"select RowID from big order by
RowID",-1,&asc,NULL);
sqlite3_prepare_v2(DB,"select RowID from big order by RowID
desc",-1,&desc,NULL);
std::cout << "Ascending" << std::endl;
for (int i=0; sqlite3_step(asc)==SQLITE_ROW; i++)
if (i%Gap==0) std::cout << FreeMBs() << std::endl;
std::cout << FreeMBs() << std::endl;
std::cout << std::endl << "Descending" << std::endl;
for (int i=0; sqlite3_step(desc)==SQLITE_ROW; i++)
if (i%Gap==0) std::cout << FreeMBs() << std::endl;
std::cout << FreeMBs() << std::endl;
sqlite3_finalize(asc);
sqlite3_finalize(desc);
sqlite3_close(DB);
getch();
return 0;
}
The big table can be emulated with this sql which creates a table with 100
million records
sqlite3_exec(DB, "create table wee as "
"with cte(a, b, c) as "
"(values (1, 'XXXXXXXXXXXXXXXXXXXX', 'XXXXXXXXXXXXXXXXXXXX') "
"union all "
"select a+1, b, c from cte where a<10000) "
"select * from cte;"
"create table big as select * from wee t1, wee t2;", 0, 0, 0);
OUTPUT
Ascending
13227
11355
9465
7582
5683
3801
Descending
3801
5868
7773
9683
11586
13473
On each run the numbers may be different but the overall results are the same.
It seems to lose RAM while step ascending and then give it back when step
descending. I can’t imagine it’s down to sqlite but can’t see anything wrong
with my code. Can anyone put me out of my misery?
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