On Wed, 17 Jan 2007, Corinna Vinschen wrote:
> On Jan 16 17:28, Brian Ford wrote:
>
> > PS: In an strace of this, I see three fstat64s called from within a
> > single mmap64. Do you know where they all are, and if two should be
> > optimized away?
>
> There's only one such call in list::set.
which is called from map::add_list, yes. I also see one in
map::get_list_by_fd.
> Your observation is strange. The first mapping, which really maps the
> file, calls fstat. The second (valid remainder) and third (sigbus area)
> mapping are anonymous mappings, which don't call fstat.
I don't think my three fstat64s correspond to your three mappings.
> In my tests, fstat64 is called only once for a file mapping.
Really? I think I see them as follows:
mmap.cc:1203 mmap64():
map_list = mmapped_areas.get_list_by_fd (fd);
mmap.cc:982 mmap_worker():
if (!(map_list = mmapped_areas.get_list_by_fd (fd))
mmap.cc:983 mmap_worker():
&& !(map_list = mmapped_areas.add_list (fd)))
Also:
mmap.cc:1134 mmap64():
DWORD low = GetFileSize (fh->get_handle (), &high);
> STC?
It looks like your statement is true for the first mmap in a process, but
subsequent mmaps have significant overhead. In the attached STC, I count
5 fstat64s of the same fd via strace, plus the GetFileSize mentioned
above.
PS. Since this isn't a bug, I don't expect you to do any more than
consider putting it in your long term low priority que or reply with a PTC
Thanks for the evaluation tuits ;-).
--
Brian Ford
Lead Realtime Software Engineer
VITAL - Visual Simulation Systems
FlightSafety International
the best safety device in any aircraft is a well-trained crew...
#include <limits.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <strings.h>
#include <string.h>
#include <fcntl.h>
#include <stdio.h>
#include <sys/mman.h>
int
main(int argc, const char *argv[])
{
char fname[PATH_MAX];
int cnt;
int fd;
struct stat sb;
void *addr;
cnt = readlink("/proc/self/exe", fname, sizeof(fname));
fname[cnt] = '\0';
#define EXE_SUFFIX ".exe"
#define STRLEN_EXE_SUFFIX (sizeof(EXE_SUFFIX)-1)
if (cnt <= STRLEN_EXE_SUFFIX
|| strcasecmp(&fname[cnt-STRLEN_EXE_SUFFIX], EXE_SUFFIX))
strcpy(fname+cnt, EXE_SUFFIX);
fd = open(fname, O_RDONLY);
if (fd < 0)
{
perror("open self");
return -1;
}
if (fstat(fd, &sb) < 0)
{
perror("stat");
return -1;
}
addr = mmap(NULL, sb.st_size, (PROT_READ|PROT_WRITE),
(MAP_NORESERVE|MAP_PRIVATE), fd, 0);
if (addr == MAP_FAILED)
{
perror("mmap1");
return -1;
}
addr = mmap(NULL, sb.st_size, (PROT_READ|PROT_WRITE),
(MAP_NORESERVE|MAP_PRIVATE), fd, 0);
if (addr == MAP_FAILED)
{
perror("mmap2");
return -1;
}
close(fd);
puts("test passed");
return 0;
}
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