linux-ext4 - Re: After unlinking a large file on ext4, the process stalls for a long time

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Date:	Thu, 17 Jul 2014 13:17:11 +0200
From:	Mason <mpeg.blue@...e.fr>
To:	Lukáš Czerner <lczerner@...hat.com>
CC:	Andreas Dilger <adilger@...ger.ca>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: After unlinking a large file on ext4, the process stalls for a
 long time

Lukáš Czerner wrote:

> So it really does not seem to be stalling in fallocate, nor unlink.
> Can you add close() before unlink, just to be sure what's happening
> there ?

Doh! Good catch! Unlinking was fast because the ref count didn't drop
to 0 on unlink, it did so on the implicit close done on exit, which
would explain why the process stalled "at the end".

If I unlink a closed file, it is indeed unlink that stalls.

[BTW, some of the e2fsprogs devs may be reading this. I suppose you
already know, but the cross-compile build was broken in 1.4.10.
I wrote a trivial patch to fix it (cf. the end of this message)
although I'm not sure I did it the canonical way.]


# time strace -T ./foo /mnt/hdd/xxx 300 2> strace.out
posix_fallocate(fd, 0, size_in_GiB << 30): 0 [412 ms]
close(fd): 0 [0 ms]
unlink(filename): 0 [111481 ms]

open("/mnt/hdd/xxx", O_WRONLY|O_CREAT|O_EXCL|O_LARGEFILE, 0600) = 3 <0.000456>
clock_gettime(CLOCK_MONOTONIC, {82152, 251657385}) = 0 <0.000085>
SYS_4320()                              = 0 <0.411628>
clock_gettime(CLOCK_MONOTONIC, {82152, 664179762}) = 0 <0.000089>
fstat64(1, {st_mode=S_IFCHR|0755, st_rdev=makedev(4, 64), ...}) = 0 <0.000094>
ioctl(1, TIOCNXCL, {B115200 opost isig icanon echo ...}) = 0 <0.000128>
old_mmap(NULL, 65536, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x773e4000 <0.000195>
write(1, "posix_fallocate(fd, 0, size_in_G"..., 54) = 54 <0.000281>
clock_gettime(CLOCK_MONOTONIC, {82152, 668413115}) = 0 <0.000077>
close(3)                                = 0 <0.000119>
clock_gettime(CLOCK_MONOTONIC, {82152, 669249479}) = 0 <0.000129>
write(1, "close(fd): 0 [0 ms]\n", 20)   = 20 <0.000145>
clock_gettime(CLOCK_MONOTONIC, {82152, 670361133}) = 0 <0.000078>
unlink("/mnt/hdd/xxx")                  = 0 <111.479283>
clock_gettime(CLOCK_MONOTONIC, {82264, 150551496}) = 0 <0.000080>
write(1, "unlink(filename): 0 [111481 ms]\n", 32) = 32 <0.000225>
exit_group(0)                           = ?

0.01user 111.48system 1:51.99elapsed 99%CPU (0avgtext+0avgdata 772maxresident)k
0inputs+0outputs (0major+434minor)pagefaults 0swaps


For reference, here's my minimal test case:

#define _FILE_OFFSET_BITS 64
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <time.h>

#define BENCH(op) do { \
  struct timespec t0; clock_gettime(CLOCK_MONOTONIC, &t0); \
  int err = op; \
  struct timespec t1; clock_gettime(CLOCK_MONOTONIC, &t1); \
  int ms = (t1.tv_sec-t0.tv_sec)*1000 + (t1.tv_nsec-t0.tv_nsec)/1000000; \
  printf("%s: %d [%d ms]\n", #op, err, ms); } while(0)

int main(int argc, char **argv)
{
  if (argc != 3) { puts("Usage: prog filename size"); return 42; }

  char *filename = argv[1];
  int fd = open(filename, O_CREAT | O_EXCL | O_WRONLY, 0600);
  if (fd < 0) { perror("open"); return 1; }

  long long size_in_GiB = atoi(argv[2]);
  BENCH(posix_fallocate(fd, 0, size_in_GiB << 30));
  BENCH(close(fd));
  BENCH(unlink(filename));
  return 0;
}


$ cat e2fsprogs-1.42.10.patch 
diff -ur a/util/Makefile.in b/util/Makefile.in
--- a/util/Makefile.in	2014-05-15 19:04:08.000000000 +0200
+++ b/util/Makefile.in	2014-07-10 15:31:04.819352596 +0200
@@ -15,7 +15,7 @@
 
 .c.o:
 	$(E) "	CC $<"
-	$(Q) $(BUILD_CC) -c $(BUILD_CFLAGS) $< -o $@
+	$(Q) $(BUILD_CC) $(CPPFLAGS) -c $(BUILD_CFLAGS) $< -o $@
 	$(Q) $(CHECK_CMD) $(ALL_CFLAGS) $<
 
 PROGS=		subst symlinks



-- 
Regards.
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