incomprehensible time consumed in using memory mapped file - c++

Question

I am writing a routine to compare two files using memory-mapped file. In case files are too big to be mapped at one go. I split the files and map them part by part. For example, to map a 1049MB file, I split it into 512MB + 512MB + 25MB.
Every thing works fine except one thing: it always take much, much longer to compare the remainder (25MB in this example), though the code logic is exactly the same. 3 observations:
it does not matter which is compared first, whether the main part (512MB * N) or the remainder (25MB in this example) comes first, the result remains the same
the extra time in the remainder seems to be spent in the user mode
Profiling in VS2010 beta 1 shows, the time is spent inside t std::_Equal(), but this function is mostly (profiler says 100%) waiting for I/O and other threads.
I tried
changing the VIEW_SIZE_FACTOR to another value
replacing the lambda functor with a member function
changing the file size under test
changing the order of execution of the remainder to before/after the loop
The result was quite consistent: it takes a lot more time in the remainder part and in the User Mode.
I suspect it has something to do with the fact that the mapped size is not a multiple of mapping alignment (64K on my system), but not sure how.
Below is the complete code for the routine and a timing measured for a 3G file.
Can anyone please explain it, Thanks?
// using memory-mapped file
template <size_t VIEW_SIZE_FACTOR>
struct is_equal_by_mmapT
{
public:
bool operator()(const path_type& p1, const path_type& p2)
{
using boost::filesystem::exists;
using boost::filesystem::file_size;
try
{
if(!(exists(p1) && exists(p2))) return false;
const size_t segment_size = mapped_file_source::alignment() * VIEW_SIZE_FACTOR;
// lanmbda
boost::function<bool(size_t, size_t)> segment_compare =
[&](size_t seg_size, size_t offset)->bool
{
using boost::iostreams::mapped_file_source;
boost::chrono::run_timer t;
mapped_file_source mf1, mf2;
mf1.open(p1, seg_size, offset);
mf2.open(p2, seg_size, offset);
if(! (mf1.is_open() && mf2.is_open())) return false;
if(!equal (mf1.begin(), mf1.end(), mf2.begin())) return false;
return true;
};
boost::uintmax_t size = file_size(p1);
size_t round = size / segment_size;
size_t remainder = size & ( segment_size - 1 );
// compare the remainder
if(remainder > 0)
{
cout << "segment size = "
<< remainder
<< " bytes for the remaining round";
if(!segment_compare(remainder, segment_size * round)) return false;
}
//compare the main part. take much less time, even
for(size_t i = 0; i < round; ++i)
{
cout << "segment size = "
<< segment_size
<< " bytes, round #" << i;
if(!segment_compare(segment_size, segment_size * i)) return false;
}
}
catch(std::exception& e)
{
cout << e.what();
return false;
}
return true;
}
};
typedef is_equal_by_mmapT<(8<<10)> is_equal_by_mmap; // 512MB
output:
segment size = 354410496 bytes for the remaining round
real 116.892s, cpu 56.201s (48.1%), user 54.548s, system 1.652s
segment size = 536870912 bytes, round #0
real 72.258s, cpu 2.273s (3.1%), user 0.320s, system 1.953s
segment size = 536870912 bytes, round #1
real 75.304s, cpu 1.943s (2.6%), user 0.240s, system 1.702s
segment size = 536870912 bytes, round #2
real 84.328s, cpu 1.783s (2.1%), user 0.320s, system 1.462s
segment size = 536870912 bytes, round #3
real 73.901s, cpu 1.702s (2.3%), user 0.330s, system 1.372s
More observations after the suggestions by responders
Further split the remainder into body and tail(remainder = body + tail), where
body = N * alignment(), and tail < 1 * alignment()
body = m * alignment(), and tail < 1 * alignment() + n * alignment(), where m is even.
body = m * alignment(), and tail < 1 * alignment() + n * alignment(), where m is exponents of 2.
body = N * alignment(), and tail = remainder - body. N is random.
the total time remains unchanged, but I can see that the time does not necessary relate to tail, but to size of body and tail. the bigger part takes more time. The time is USER TIME, which is most incomprehensible to me.
I also look at the pages faults through Procexp.exe. the remainder does NOT take more faults than the main loop.
Updates 2
I've performed some test on other workstations, and it seem the issue is related to the hardware configuration.
Test Code
// compare the remainder, alternative way
if(remainder > 0)
{
//boost::chrono::run_timer t;
cout << "Remainder size = "
<< remainder
<< " bytes \n";
size_t tail = (alignment_size - 1) & remainder;
size_t body = remainder - tail;
{
boost::chrono::run_timer t;
cout << "Remainder_tail size = " << tail << " bytes";
if(!segment_compare(tail, segment_size * round + body)) return false;
}
{
boost::chrono::run_timer t;
cout << "Remainder_body size = " << body << " bytes";
if(!segment_compare(body, segment_size * round)) return false;
}
}
Observation:
On another 2 PCs with the same h/w configurations with mine, the result is consistent as following:
------VS2010Beta1ENU_VSTS.iso [1319909376 bytes] ------
Remainder size = 44840960 bytes
Remainder_tail size = 14336 bytes
real 0.060s, cpu 0.040s (66.7%), user 0.000s, system 0.040s
Remainder_body size = 44826624 bytes
real 13.601s, cpu 7.731s (56.8%), user 7.481s, system 0.250s
segment size = 67108864 bytes, total round# = 19
real 172.476s, cpu 4.356s (2.5%), user 0.731s, system 3.625s
However, running the same code on a PC with a different h/w configuration yielded:
------VS2010Beta1ENU_VSTS.iso [1319909376 bytes] ------
Remainder size = 44840960 bytes
Remainder_tail size = 14336 bytes
real 0.013s, cpu 0.000s (0.0%), user 0.000s, system 0.000s
Remainder_body size = 44826624 bytes
real 2.468s, cpu 0.188s (7.6%), user 0.047s, system 0.141s
segment size = 67108864 bytes, total round# = 19
real 65.587s, cpu 4.578s (7.0%), user 0.844s, system 3.734s
System Info
My workstation yielding imcomprehensible timing:
OS Name: Microsoft Windows XP Professional
OS Version: 5.1.2600 Service Pack 3 Build 2600
OS Manufacturer: Microsoft Corporation
OS Configuration: Member Workstation
OS Build Type: Uniprocessor Free
Original Install Date: 2004-01-27, 23:08
System Up Time: 3 Days, 2 Hours, 15 Minutes, 46 Seconds
System Manufacturer: Dell Inc.
System Model: OptiPlex GX520
System type: X86-based PC
Processor(s): 1 Processor(s) Installed.
[01]: x86 Family 15 Model 4 Stepping 3 GenuineIntel ~2992 Mhz
BIOS Version: DELL - 7
Windows Directory: C:\WINDOWS
System Directory: C:\WINDOWS\system32
Boot Device: \Device\HarddiskVolume2
System Locale: zh-cn;Chinese (China)
Input Locale: zh-cn;Chinese (China)
Time Zone: (GMT+08:00) Beijing, Chongqing, Hong Kong, Urumqi
Total Physical Memory: 3,574 MB
Available Physical Memory: 1,986 MB
Virtual Memory: Max Size: 2,048 MB
Virtual Memory: Available: 1,916 MB
Virtual Memory: In Use: 132 MB
Page File Location(s): C:\pagefile.sys
NetWork Card(s): 3 NIC(s) Installed.
[01]: VMware Virtual Ethernet Adapter for VMnet1
Connection Name: VMware Network Adapter VMnet1
DHCP Enabled: No
IP address(es)
[01]: 192.168.75.1
[02]: VMware Virtual Ethernet Adapter for VMnet8
Connection Name: VMware Network Adapter VMnet8
DHCP Enabled: No
IP address(es)
[01]: 192.168.230.1
[03]: Broadcom NetXtreme Gigabit Ethernet
Connection Name: Local Area Connection 4
DHCP Enabled: Yes
DHCP Server: 10.8.0.31
IP address(es)
[01]: 10.8.8.154
Another workstation yielding "correct" timing:
OS Name: Microsoft Windows XP Professional
OS Version: 5.1.2600 Service Pack 3 Build 2600
OS Manufacturer: Microsoft Corporation
OS Configuration: Member Workstation
OS Build Type: Multiprocessor Free
Original Install Date: 5/18/2009, 2:28:18 PM
System Up Time: 21 Days, 5 Hours, 0 Minutes, 49 Seconds
System Manufacturer: Dell Inc.
System Model: OptiPlex 755
System type: X86-based PC
Processor(s): 1 Processor(s) Installed.
[01]: x86 Family 6 Model 15 Stepping 13 GenuineIntel ~2194 Mhz
BIOS Version: DELL - 15
Windows Directory: C:\WINDOWS
System Directory: C:\WINDOWS\system32
Boot Device: \Device\HarddiskVolume1
System Locale: zh-cn;Chinese (China)
Input Locale: en-us;English (United States)
Time Zone: (GMT+08:00) Beijing, Chongqing, Hong Kong, Urumqi
Total Physical Memory: 3,317 MB
Available Physical Memory: 1,682 MB
Virtual Memory: Max Size: 2,048 MB
Virtual Memory: Available: 2,007 MB
Virtual Memory: In Use: 41 MB
Page File Location(s): C:\pagefile.sys
NetWork Card(s): 3 NIC(s) Installed.
[01]: Intel(R) 82566DM-2 Gigabit Network Connection
Connection Name: Local Area Connection
DHCP Enabled: Yes
DHCP Server: 10.8.0.31
IP address(es)
[01]: 10.8.0.137
[02]: VMware Virtual Ethernet Adapter for VMnet1
Connection Name: VMware Network Adapter VMnet1
DHCP Enabled: Yes
DHCP Server: 192.168.154.254
IP address(es)
[01]: 192.168.154.1
[03]: VMware Virtual Ethernet Adapter for VMnet8
Connection Name: VMware Network Adapter VMnet8
DHCP Enabled: Yes
DHCP Server: 192.168.2.254
IP address(es)
[01]: 192.168.2.1
Any explanation theory? Thanks.

Answer 1

This behavior looks quite illogical. I wonder what would happen if we tried something stupid. Provided the overall file is larger than 512MB you could compare again a full 512MB for the last part instead of the remaining size.
something like:
if(remainder > 0)
{
cout << "segment size = "
<< remainder
<< " bytes for the remaining round";
if (size > segment_size){
block_size = segment_size;
offset = size - segment_size;
}
else{
block_size = remainder;
offset = segment_size * i
}
if(!segment_compare(block_size, offset)) return false;
}
It seems a really dumb thing to do because we would be comparing part of the file two times but if your profiling figures are accurate it should be faster.
It won't give us an answer (yet) but if it is indeed faster it means the response we are looking for lies in what your program does for small blocks of data.

Answer 2

How fragmented is the file you are comparing with? You can use FSCTL_GET_RETRIEVAL_POINTERS to get the ranges that the file maps to on disk. I suspect the last 25 MB will have a lot of small ranges to account for the performance you have measured.

Answer 3

I wonder if mmap behaves strangely when a segment isn't an even number of pages in size? Maybe you can try handling the last parts of the file by progressively halving your segment sizes until you get to a size that's less than mapped_file_source::alignment() and handling that last little bit specially.
Also, you say you're doing 512MB blocks, but your code sets the size to 8<<10. It then multiplies that by mapped_file_source::alignment(). Is mapped_file_source::alignment() really 65536?
I would recommend, to be more portable and cause less confusion, that you simply use the size as given in the template parameter and simply require that it be an even multiple of mapped_file_source::alignment() in your code. Or have people pass in the power of two to start at for the block size, or something. Having the block size passed in as a template parameter then be multiplied by some strange implementation defined constant seems a little odd.

Answer 4

I know this isn't an exact answer to your question; but have you tried side-stepping the entire problem - i.e. just map the entire file in one go?
I know little about Win32 memory management; but on Linux you can use the MAP_NORESERVE flag with mmap(), so you don't need to reserve RAM for the entire filesize. Considering you are just reading from both files the OS should be able to throw away pages at any time if it gets short of RAM...

Answer 5

I would try it on a Linux or BSD just to see how it acts, out of curiousity.
I have a really rough guess about the problem:
I bet that Windows is doing a lot of extra checks to make sure it doesn't map past the end of the file. In the past there have been security problems in some OS's that allowed a mmap user to view filesystem-private data or data from other files in the area just past the end of the map, so being careful here is a good idea for a OS designer. So Windows may be using a much more careful "copy data from disk to kernel, zero out unmapped data, copy data to user" instead of the much faster "copy data from disk to user".
Try mapping to just under the end of the file, excluding the last bytes that don't fit into a 64K block.

Answer 6

Could it be that a virus scanner is causing these strange results? Have you tried without virus scanner?
Regards,
Sebastiaan