Re: Why is network reading slow?

From:
"Tom Serface" <tom@camaswood.com>
Newsgroups:
microsoft.public.vc.mfc
Date:
Fri, 15 Jan 2010 15:48:08 -0800
Message-ID:
<#8rq00jlKHA.5128@TK2MSFTNGP05.phx.gbl>
Yeah, I confess in my case I was more concerned with speed of the copy over
number of reads and writes. I need to restore data from optical media as
fast as possible and Windows is not very efficient at reading removable
media.

Tom

"Joseph M. Newcomer" <newcomer@flounder.com> wrote in message
news:qnc1l5pa33uo1lv0050rik345hp3sq31a1@4ax.com...

50MB is about 2% of the available 2GB address space. Not even worth
discussing. Things
don't get interesting until you start getting an order of magnitude
larger, at least.
joe

On Thu, 14 Jan 2010 22:40:19 -0800, "Tom Serface" <tom@camaswood.com>
wrote:

I think you need to be really careful not to use up all the real memory as
well so that you don't start swapping to disk. That is a killer that you
don't even see coming, although 50MB shouldn't be a problem on most modern
computers.

Tom

"Joseph M. Newcomer" <newcomer@flounder.com> wrote in message
news:686vk59l8chun24uceekvdc8pt2uj4n811@4ax.com...

By the way, did anyone really notice that ReadFile and WriteFile in
Win64
cannot read or
write more than 4.2GB? Seems really, really strange the length and
bytes
read did not
become DWORD_PTR values...
joe

On Thu, 14 Jan 2010 16:37:26 -0500, Joseph M. Newcomer
<newcomer@flounder.com> wrote:

Yes, but the file size was given as 50MB.
joe

On Thu, 14 Jan 2010 14:24:30 -0600, Stephen Myers
<""StephenMyers\"@discussions@microsoft.com"> wrote:

Just to verify my (admittedly limited) understanding...

I assume that the code posted will fail for files greater than 2GB or
so
with a 32 bit OS due to available address space.

Steve

Joseph M. Newcomer wrote:

See below...
On Thu, 14 Jan 2010 09:01:47 -0600, "Peter Olcott"
<NoSpam@SeeScreen.com> wrote:

"Hector Santos" <sant9442@nospam.gmail.com> wrote in message
news:OzySgEPlKHA.2132@TK2MSFTNGP05.phx.gbl...

Peter Olcott wrote:

"Hector Santos" <sant9442@nospam.gmail.com> wrote in
message news:%23OQCOfNlKHA.1824@TK2MSFTNGP04.phx.gbl...

Peter Olcott wrote:

By File Copy, you mean DOS copy command or the
CopyFile() API?

I am using the DOS command prompt's copy command. This
is fast.

The problem is the contradiction formed by the fact that
reading and writng the file is fast, while reading and
not wrting this same file is slow.
I am currently using fopen() and fread(); I am using
Windows XP.

True, if the DOS copy command is fast,then I believe the
code you are using is not optimal. The DOS Copy is using
the same CreateFile() API which fopen() also finally uses
in the RTL. So you should be able to match the same
performance of the DOS Copy command.

Have you tried using setvbuf to set a buffer cache?

Here is a small test code that opens a 50 meg file:

// File: V:\wc7beta\testbufsize.cpp
// Compile with: cl testbufsize.cpp

#include <stdio.h>
#include <windows.h>

void main(char argc, char *argv[])
{
   char _cache[1024*16] = {0}; // 16K cache
   BYTE buf[1024*1] = {0}; // 1K buffer

****
Reading a 50MB file, why such an incredibly tiny buffer?
****

   FILE *fv = fopen("largefile.dat","rb");
   if (fv) {
       int res = setvbuf(fv, _cache, _IOFBF,
sizeof(_cache));
       DWORD nTotal = 0;
       DWORD nDisks = 0;
       DWORD nLoops = 0;
       DWORD nStart = GetTickCount();
       while (!feof(fv)) {
            nLoops++;
            memset(&buf,sizeof(buf),0);

****
The memset is silly. Wastes time, accomplishes nothing. You are
setting a buffer to 0
right before completely overwriting it! This is like writing
int a;

a = 0; // make sure a is 0 before assigning b
a = b;
****

            int nRead = fread(buf,1,sizeof(buf),fv);
            nTotal +=nRead;
            if (nRead > 0 && !fv->_cnt) nDisks++;
       }
       fclose(fv);
       printf("Time: %d | Size: %d | Reads: %d | Disks:
%d\n",
               GetTickCount()-nStart,
               nTotal,
               nLoops,
               nDisks);
    }
}

****
If I were reading a small 50MB file, I would do

void tmain(int argc, _TCHAR * argv[])
   {
    HANDLE h = CreateFile(_T("largefile.dat"), GENERIC_READ, 0, NULL,
OPEN_EXISTING,
FILE_ATTRIBUTE_NORMAL, NULL);

    LARGE_INTEGER size;

    GetFileSizeEx(h, &size);

    // This code assumes file is < 4.2GB!
    LPVOID p = VirtualAlloc(NULL, (SIZE_T)size.LowPart, MEM_COMMIT,
PAGE_READWRITE);
    DWORD bytesRead;
    ReadFile(h, p, size.LowPart, &bytesRead, NULL);
    ... process data
    VirtualFree(p, (SIZE_T)size.LowPart, MEM_DECOMMIT);
    return 0;
   }

Note that the above does not do any error checking; the obvious error
checking is left as
an Exercise For The Reader. No read loops, no gratuitous memsets,
just
simple code that
does exactly ONE ReadFile.
joe

What this basically shows is the number of disk hits it
makes
by checking the fv->_cnt value. It shows that as long as
the cache size is larger than the read buffer size, you
get the same number of disk hits. I also spit out the
milliseconds. Subsequent runs, of course, is faster since
the OS API CreateFile() is used by the RTL in buffer mode.

Also do you know what protocol you have Samba using?

I am guessing that the code above will work with a file of
any size?
If that is the case, then you solved my problem.
The only Samba protocol that I am aware of is smb.

--
HLS

Joseph M. Newcomer [MVP]
email: newcomer@flounder.com
Web: http://www.flounder.com
MVP Tips: http://www.flounder.com/mvp_tips.htm

Joseph M. Newcomer [MVP]
email: newcomer@flounder.com
Web: http://www.flounder.com
MVP Tips: http://www.flounder.com/mvp_tips.htm

Joseph M. Newcomer [MVP]
email: newcomer@flounder.com
Web: http://www.flounder.com
MVP Tips: http://www.flounder.com/mvp_tips.htm

Joseph M. Newcomer [MVP]
email: newcomer@flounder.com
Web: http://www.flounder.com
MVP Tips: http://www.flounder.com/mvp_tips.htm

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