fread + journaling = slow?

4 Oct 2004

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Folk:

To make a *very* long story as short as I can, I am running into a
performance issue that has me scratching my head a little bit (and might
save somebody else some time in the future when list searching reappears).

The issue:  when journaling is on, I see poor fread() performance.
Shark:      all time is spent in semaphores in the kernel.

poor == 10x slower.

The code in question (see below) creates the following output:

***

Moe:~/proj/journal pdc$ time ./tj /{pathcut}/test.data
file size:      1984 kb (1.9375 meg)
total reads:    2031616

real    0m4.880s
user    0m0.120s
sys     0m1.130s

***

Do nothing else but this, using Disk Utility:

  Turn Journaling Off on data file volume.
  Unmount Volume.
  Mount Volume.

***

Moe:~/proj/journal pdc$ time ./tj /{pathcut}test.data
file size:      1984 kb (1.9375 meg)
total reads:    2031616

real    0m0.413s
user    0m0.070s
sys     0m0.240s

***

4.9s vs 0.4s

And this extra 4.4s is not a constant time -- it's at least linear.

So, what gives?  Is this normal behaviour?  Any Ideas?

Yes, I have re-implemented my app to use mmap(), but issues I've run into
with that are forcing me to re-evaluate the original issue: slow fread()
when Journaling is on.

Thanks for any ideas and sorry so verbose,

-pdc

--- snip ---

/* compiled with "cc -o jt jt.c" */

#include <stdio.h>

static void scan(const char *fname);

int main(int argc, char *argv[])
{
    if (argc != 2) {
        printf("Usage: %s file\n", argv[0]);
        exit(1);
    }

    scan(argv[1]);
    return 0;
}

#define READ_SIZE   4096

void scan(const char *fname)
{
    FILE    *fptr   = fopen(fname, "rb");
    long    size    = 0;
    char    buff[READ_SIZE];
    long    idx;

    if (!fptr) {
        printf("can't open '%s'\n", fname);
        exit(1);
    }

    fseek(fptr, 0, SEEK_END);
    size = ftell(fptr);
    fseek(fptr, 0, SEEK_SET);

    printf("file size:\t%ld kb (%g meg)\n", size/1024,
(float)size/(1024.0*1024.0));

    for (idx = 0; idx < size/128 ; idx++) {
        long pos = rand() % (size - READ_SIZE);
        pos        &= 0xFFFF1000; // benefit of the doubt...
        fseek(fptr, pos, SEEK_SET);

        if (fread(buff, READ_SIZE, 1, fptr) != 1) {
            printf("cant read at #%ld (pos: %ld)\n", idx, pos);
            exit(1);
        }
    }
    printf("total reads:\t%ld\n", size);

    fclose(fptr);
}

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fread + journaling = slow?

Paul Carnine