title: Writing multiple files slows down after x seconds
body: <p>I have code which gets frames from a camera and then saves it to disk. The structure of the code is: multiple threads malloc and copy their frames into new memory, enqueue memory. Finally, another thread removes frames from queue and writes them (using ffmpeg API, raw video no compression) to their files (actually I'm using my own memory pool so malloc is only called when I need more buffers). I can have upto 8 files/cams open at the same time enqueing. </p>
  
  <p>The problem is that for the first 45 sec everything works fine: there's never more than one frame on queue. But after that my queue gets backed up, processing takes just a few ms longer resulting in increased ram usage because I cannot save the frames fast enough so I have to malloc more memory to store them. </p>
  
  <p>I have a 8 core, 16GB RAM Windows 7 64 bit computer (NTFS, lots of free space in second disk drive). The disk is supposed to be able to write upto 6Gbits/sec. To save my data in time I need to be able to write at 50 MB/sec. I tested disk speed with "PassMark PerformanceTest" and I had 8 threads writing files simultaneously exactly like ffmpeg saves files (synchronized, uncached I/O) and it was able to achieve 100MB/sec. So why isn't my writes able to achieve that?</p>
  
  <p>Here's how the ffmpeg writes look from Process monitor logs:  </p>
  
  <pre> 
  Time of Day     Operation   File#   Result  Detail
  2:30:32.8759350 PM  WriteFile   8   SUCCESS Offset: 749,535,120, Length: 32,768
  2:30:32.8759539 PM  WriteFile   8   SUCCESS Offset: 749,567,888, Length: 32,768
  2:30:32.8759749 PM  WriteFile   8   SUCCESS Offset: 749,600,656, Length: 32,768
  2:30:32.8759939 PM  WriteFile   8   SUCCESS Offset: 749,633,424, Length: 32,768
  2:30:32.8760314 PM  WriteFile   8   SUCCESS Offset: 749,666,192, Length: 32,768
  2:30:32.8760557 PM  WriteFile   8   SUCCESS Offset: 749,698,960, Length: 32,768
  2:30:32.8760866 PM  WriteFile   8   SUCCESS Offset: 749,731,728, Length: 32,768
  2:30:32.8761259 PM  WriteFile   8   SUCCESS Offset: 749,764,496, Length: 32,768
  2:30:32.8761452 PM  WriteFile   8   SUCCESS Offset: 749,797,264, Length: 32,768
  2:30:32.8761629 PM  WriteFile   8   SUCCESS Offset: 749,830,032, Length: 32,768
  2:30:32.8761803 PM  WriteFile   8   SUCCESS Offset: 749,862,800, Length: 32,768
  2:30:32.8761977 PM  WriteFile   8   SUCCESS Offset: 749,895,568, Length: 32,768
  2:30:32.8762235 PM  WriteFile   8   SUCCESS Offset: 749,928,336, Length: 32,768, Priority: Normal
  2:30:32.8762973 PM  WriteFile   8   SUCCESS Offset: 749,961,104, Length: 32,768
  2:30:32.8763160 PM  WriteFile   8   SUCCESS Offset: 749,993,872, Length: 32,768
  2:30:32.8763352 PM  WriteFile   8   SUCCESS Offset: 750,026,640, Length: 32,768
  2:30:32.8763502 PM  WriteFile   8   SUCCESS Offset: 750,059,408, Length: 32,768
  2:30:32.8763649 PM  WriteFile   8   SUCCESS Offset: 750,092,176, Length: 32,768
  2:30:32.8763790 PM  WriteFile   8   SUCCESS Offset: 750,124,944, Length: 32,768
  2:30:32.8763955 PM  WriteFile   8   SUCCESS Offset: 750,157,712, Length: 32,768
  2:30:32.8764072 PM  WriteFile   8   SUCCESS Offset: 750,190,480, Length: 4,104
  2:30:32.8848241 PM  WriteFile   4   SUCCESS Offset: 750,194,584, Length: 32,768
  2:30:32.8848481 PM  WriteFile   4   SUCCESS Offset: 750,227,352, Length: 32,768
  2:30:32.8848749 PM  ReadFile    4   END OF FILE Offset: 750,256,128, Length: 32,768, I/O Flags: Non-cached, Paging I/O, Synchronous Paging I/O, Priority: Normal
  2:30:32.8848989 PM  WriteFile   4   SUCCESS Offset: 750,260,120, Length: 32,768
  2:30:32.8849157 PM  WriteFile   4   SUCCESS Offset: 750,292,888, Length: 32,768
  2:30:32.8849319 PM  WriteFile   4   SUCCESS Offset: 750,325,656, Length: 32,768
  2:30:32.8849475 PM  WriteFile   4   SUCCESS Offset: 750,358,424, Length: 32,768
  2:30:32.8849637 PM  WriteFile   4   SUCCESS Offset: 750,391,192, Length: 32,768
  2:30:32.8849880 PM  WriteFile   4   SUCCESS Offset: 750,423,960, Length: 32,768, Priority: Normal
  2:30:32.8850400 PM  WriteFile   4   SUCCESS Offset: 750,456,728, Length: 32,768
  2:30:32.8850727 PM  WriteFile   4   SUCCESS Offset: 750,489,496, Length: 32,768, Priority: Normal
  </pre>
  
  <p>This looks very efficient, however, from DiskMon the actual disk writes look ridiculously fragmented back and forth writing which may account for this slow speed. See the graph for the write speed according to this data (~5MB/s).</p>
  
  <pre> TIme  Write duration  Sector      Length  MB/sec
  95.6    0.00208855  1490439632  896 0.409131784
  95.6    0.00208855  1488197000  128 0.058447398
  95.6    0.00009537  1482323640  128 1.279965529
  95.6    0.00009537  1482336312  768 7.679793174
  95.6    0.00009537  1482343992  384 3.839896587
  95.6    0.00009537  1482350648  768 7.679793174
  95.6    0.00039101  1489278984  1152    2.809730729
  95.6    0.00039101  1489393672  896 2.185346123
  95.6    0.0001812   1482349368  256 1.347354443
  95.6    0.0001812   1482358328  896 4.715740549
  95.6    0.0001812   1482370616  640 3.368386107
  95.6    0.0001812   1482378040  256 1.347354443
  95.6    0.00208855  1488197128  384 0.175342193
  95.6    0.00208855  1488202512  640 0.292236989
  95.6    0.00208855  1488210320  1024    0.467579182
  95.6    0.00009537  1482351416  256 2.559931058
  95.6    0.00009537  1482360120  896 8.959758703
  95.6    0.00009537  1482371896  640 6.399827645
  95.6    0.00009537  1482380088  256 2.559931058
  95.7    0.00039101  1489394568  1152    2.809730729
  95.7    0.00039101  1489396744  352 0.858528834
  95.7    0.00039101  1489507944  544 1.326817289
  95.7    0.0001812   1482378296  768 4.042063328
  95.7    0.0001812   1482392120  768 4.042063328
  95.7    0.0001812   1482400568  512 2.694708885
  95.7    0.00208855  1488224144  768 0.350684386
  95.7    0.00208855  1488232208  384 0.175342193
  </pre>
  
  <p><img src="https://i.stack.imgur.com/0ukCL.png" alt="enter image description here"></p>
  
  <p>I'm pretty confident it's not my code, because I timed everything and for example enqueing takes a few us suggesting that threads don't get stuck waiting for each other. It must be the disk writes. So the question is how can I improve my disk writes and what can I do to profile actual disk writes (remember that I rely on FFmpeg dlls to save so I cannot access the low level writing functions directly). If I cannot figure it out, I'll dump all the frames in a single sequential binary file (which should increase I/O speed) and then split it into video files post processing.</p>
  
  <p>I don't know how much my disk I/O is caching (CacheSet only shows disk C cache size), but the following image from the performance monitor taken at 0 and 45 sec into the video (just before my queue starts piling up) looks weird to me. Basically, the modified set and standby set grew from very little to this large value. Is that the data being cached? Is it possible that only at 45 sec data is starting to be written to disk so suddenly everything slows down?
  <img src="https://i.stack.imgur.com/GaeoA.png" alt="enter image description here"><img src="https://i.stack.imgur.com/ha4IO.png" alt="enter image description here"> 
  (FYI, LabVIEW is the program that loads my dll.)</p>
  
  <p>I'll appreciate any help.<br>
  M.</p>