Messages in this thread | | | Date | Fri, 5 Dec 2003 15:00:08 -0600 | From | Andy Isaacson <> | Subject | sparse file performance (was Re: Is there a "make hole" (truncate in middle) syscall?) |
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On Thu, Dec 04, 2003 at 05:23:48PM -0600, Andy Isaacson wrote: > On Thu, Dec 04, 2003 at 02:32:23PM -0600, Rob Landley wrote: > > What are the downsides of holes? (How big do they have to be to > > actually save space, is there a performance penalty to having a file > > with 1000 4k holes in it, etc...) > > It's filesystem-dependent; some filesystems don't implement sparse > files. The lower bound is one block; on extents-based filesystems like > XFS it might be bigger. (If you've got 1GB of data, then a 1MB block of > zeros, then another GB of data, you're probably better off allocating a > single 2GB extent rather than two smaller extents with a hole.) > > There's no inherent downside to holey files; in fact they can be a > straight-up performance win -- that's a block that doesn't need to be > read from disk, just hand the user a COW pointer to your zero page. And > if you're lucky and the preceding and following blocks are allocated > adjacent on disk, you can do it all as a single streaming IO.
I got curious enough to run some tests, and was suprised at the results. My machine (Athlon XP 2400+, 2030 MHz, 512 MB, KT400, 2.4.22) can read out of buffer cache at 234 MB/s, and off of its IDE disk at 40 MB/s. I'd assumed that read(2)ing a holey file would go faster than reading out of buffer cache; in theory you could do it completely in L1 cache (with a 4KB buffer, it's just a ton of syscalls, some page table manipulation, and a bunch of memcpy() out of a single zero page). But it turns out that reading a hole is *slower* than reading data from buffer cache, just 195 MB/s.
200 MB file 234 MB/s (with warm caches) 1 GB file 40 MB/s (exceeds physical memory) 1 GB sparse file 195 MB/s
the 1GB sparse file was created with "dd if=file of=1gsparse bs=1M count=1 seek=1023"; the filesystem is ext3.
Here's 'vmstat 5' while reading the 200MB file in a loop:
procs -----------memory---------- ---swap-- -----io---- --system-- ----cpu---- r b swpd free buff cache si so bi bo in cs us sy id wa
1 0 50968 4468 4872 410424 0 0 0 9 102 46 62 38 0 0 1 0 50968 4448 4892 410424 0 0 0 6 101 41 62 38 0 0 1 0 50968 4428 4912 410424 0 0 0 6 101 40 62 38 0 0 1 0 50968 4404 4936 410424 0 0 0 6 101 37 61 39 0 0 1 0 50968 4384 4956 410424 0 0 0 8 105 117 60 40 0 0 1 0 50968 4484 4984 410296 0 0 0 9 103 81 62 38 0 0
here's 'vmstat 5' while reading the 1GB sparse file in a loop:
1 0 55448 4460 2464 417320 0 0 217 6 144 3117 45 49 6 0 1 0 55448 4444 2480 417304 0 0 219 6 204 3237 50 44 6 0 1 0 55448 4444 2488 417288 0 0 218 9 181 3200 49 45 6 0 1 0 55460 4456 2468 417140 30 0 249 6 182 3193 46 48 6 0 1 0 55460 4396 2484 417300 0 2 220 12 140 3084 46 48 6 0 1 0 55460 4356 2464 417360 0 0 216 2 145 3101 47 48 6 0
The code is simply doing
while((n = read(fd, buf, sizeof(buf))) > 0) { c += n; for(i=0; i < n; i++) { hist[buf[i]]++; } }
compiled with gcc 3.3.2 -O2.
Code appended.
-andy
#include <stdio.h> #include <stdarg.h> #include <stdlib.h>
#include <unistd.h> #include <errno.h> #include <string.h> #include <sys/time.h> #include <fcntl.h> #include <ctype.h>
static void die(char *fmt, ...) { va_list a; va_start(a, fmt); vfprintf(stderr, fmt, a); va_end(a); exit(1); }
double tod(void) { static struct timeval tv1; struct timeval tv2; double r;
if(tv1.tv_sec == 0) { gettimeofday(&tv1, 0); return 0; } gettimeofday(&tv2, 0); r = (tv2.tv_sec - tv1.tv_sec) + (tv2.tv_usec - tv1.tv_usec) / 1e6; memcpy(&tv1, &tv2, sizeof(tv1)); return r; }
int main(int argc, char **argv) { char buf[4096]; int fd, i, n, m; long long c = 0; double t1, t2; int hist[256] = { 0 }; unsigned char *p = buf;
if(argc != 2) die("usage: %s file\n", argv[0]);
if((fd = open(argv[1], O_RDONLY)) == -1) die("%s: %s\n", argv[1], strerror(errno));
t1 = tod(); while((n = read(fd, buf, sizeof(buf))) > 0) { c += n; for(i=0; i < n; i++) { hist[p[i]]++; } } t2 = tod(); if(n == -1) die("read: %s\n", strerror(errno));
m = 0; for(i=1; i<256; i++) if(hist[i] > hist[m]) m = i; printf("%lld characters read, mode at %d '%c' with %d\n", c, m, isprint(m) ? m : '?', hist[m]); printf("%f seconds, %f MB/sec\n", t2-t1, c / (t2-t1) / 1e6); return 0; } - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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