Erratic performance of IO operations in Qt4 program

[jcox23]

Hi,

Somewhere in a qt4 program, I launch a thread, that will do some IO operations in a file...
I'm trying to time this operation...

QTime t;
t.start();
 
 SegDIO_getRMS(mFilename.data(), mSegdAttributes->averaged, mSegdAttributes);
 
 qDebug("Time to extract datasquare: %d ms", t.elapsed());

QTime t;
t.start();

 SegDIO_getRMS(mFilename.data(), mSegdAttributes->averaged, mSegdAttributes);

 qDebug("Time to extract datasquare: %d ms", t.elapsed());

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all three parameters of the function SegDIO_getRMS are pointers... (to char*, float* and pointer to a custom struct....)

Now there is a timer also in the called function, wrapping all operations...

At the beginning, the 2 timers are in agreement... everything's as fast as it should...but after some time,
the outside time is reporting times that are 5 times bigger than the inside timer...

Unless I'n not understanding something properly , the only operations made between these 2 timers are
locating the position in memory of the function and copy 3 pointers to it... that is reported during almost a full second...
which makes no sense...

I'm out of ideas of how to diagnostic the problem...

Does somebody has an idea of what's going on?

EDIT: additional info:
monitoring the I/O with iostat confirms that something goes to hell when timing become inconsistent :

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
sda              26.50       232.00        16.00        464         32
sr0               0.00         0.00         0.00          0          0
 
Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
sda              14.50       120.00        16.00        240         32
sr0               0.00         0.00         0.00          0          0
 
Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
sda            1578.50     30748.00         0.00      61496          0
sr0               0.00         0.00         0.00          0          0
 
Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
sda            3230.50     63684.00        28.00     127368         56
sr0               0.00         0.00         0.00          0          0

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
sda              26.50       232.00        16.00        464         32
sr0               0.00         0.00         0.00          0          0

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
sda              14.50       120.00        16.00        240         32
sr0               0.00         0.00         0.00          0          0

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
sda            1578.50     30748.00         0.00      61496          0
sr0               0.00         0.00         0.00          0          0

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
sda            3230.50     63684.00        28.00     127368         56
sr0               0.00         0.00         0.00          0          0

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