close() slow on socketpairs?

[Dan Kegel]

I wrote a little benchmark to call either pipe() or socketpair() 
8000 times, then close() on all the fds produced by pipe or socketpair.
On both 2.2.14 and 2.2.16, pipe and socketpair are nice and speedy.
close() is fine for pipes, but at 8000 socketpairs, each call to close() 
takes 14 *milliseconds* at 100% cpu usage.  What's up with that?

- Dan

#include <stdio.h>
#include <unistd.h>
#include <time.h>
#include <sys/types.h>
#include <sys/socket.h>
main()
{
        int i;
        int fds[16000];
        int n = 8000;
        time_t start = time(0);
        for (i=0; i<n; i+= 2)
                if (socketpair(AF_LOCAL, SOCK_STREAM, 0, fds + i))
                        exit(1);
        printf("Opening %d socketpairs took %d seconds\n", n, time(0) - start);
        start = time(0);
        for (i=0; i<n; i+= 2)
                if (close(fds[i]) || close(fds[i+1]))
                        exit(1);
        printf("Closing %d pipes took %d seconds\n", n, time(0) - start);
}
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