[Libguestfs] [PATCH 4/4] lib: qemu: Memoize qemu feature detection.
Richard W.M. Jones
rjones at redhat.com
Thu May 12 21:26:25 UTC 2016
qemu feature detection takes about 95ms on my laptop. The overhead is
almost all due to the time taken by the glibc link loader opening the
170+ libraries that qemu is linked to (×2 because we need to run qemu
twice).
Fixing that is seriously hard work.
Therefore memoize the results of guestfs_int_test_qemu.
This is keyed on the size and mtime of the qemu binary, so if the user
changes the qemu binary (eg. setting LIBGUESTFS_HV) we discard the
memoized result and rerun the qemu commands. There is also a
generation number so we can bump the generation in future versions of
libguestfs to invalidate all previously cached data.
The memo is stored in the supermin cache directory (eg. /var/tmp/.guestfs-*)
in the files:
qemu.stat Result of 'stat(2)' of the qemu binary
qemu.help qemu -help output
qemu.devices qemu -devices ? output
Note these files are only stored when using the 'direct' backend. For
the libvirt backend, libvirt itself memoizes this data in its own
place.
---
src/qemu.c | 154 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 146 insertions(+), 8 deletions(-)
diff --git a/src/qemu.c b/src/qemu.c
index feb8bd6..259a6fd 100644
--- a/src/qemu.c
+++ b/src/qemu.c
@@ -58,23 +58,163 @@ struct qemu_data {
guestfs_int_qemu_supports_virtio_scsi */
};
+static int test_qemu (guestfs_h *g, struct qemu_data *data);
static void parse_qemu_version (guestfs_h *g, struct qemu_data *data);
static void read_all (guestfs_h *g, void *retv, const char *buf, size_t len);
+/* This is saved in the qemu.stat file, so if we decide to change the
+ * test_qemu memoization format/data in future, we should increment
+ * this to discard any memoized data cached by previous versions of
+ * libguestfs.
+ */
+#define MEMO_GENERATION 1
+
/**
* Test qemu binary (or wrapper) runs, and do C<qemu -help> so we know
* the version of qemu what options this qemu supports, and
* C<qemu -device ?> so we know what devices are available.
+ *
+ * This caches the results in the cachedir so that as long as the qemu
+ * binary does not change, calling this is effectively free.
*/
struct qemu_data *
guestfs_int_test_qemu (guestfs_h *g)
{
+ struct qemu_data *data;
+ struct stat statbuf;
+ CLEANUP_FREE char *cachedir = NULL, *qemu_stat_filename = NULL,
+ *qemu_help_filename = NULL, *qemu_devices_filename = NULL;
+ FILE *fp;
+ int generation;
+ uint64_t prev_size, prev_mtime;
+
+ if (stat (g->hv, &statbuf) == -1) {
+ perrorf (g, "stat: %s", g->hv);
+ return NULL;
+ }
+
+ cachedir = guestfs_int_lazy_make_supermin_appliance_dir (g);
+ if (cachedir == NULL)
+ return NULL;
+
+ qemu_stat_filename = safe_asprintf (g, "%s/qemu.stat", cachedir);
+ qemu_help_filename = safe_asprintf (g, "%s/qemu.help", cachedir);
+ qemu_devices_filename = safe_asprintf (g, "%s/qemu.devices", cachedir);
+
+ /* Did we previously test the same version of qemu? */
+ debug (g, "checking for previously cached test results of %s, in %s",
+ g->hv, cachedir);
+
+ fp = fopen (qemu_stat_filename, "r");
+ if (fp == NULL)
+ goto do_test;
+ if (fscanf (fp, "%d %" SCNu64 " %" SCNu64,
+ &generation, &prev_size, &prev_mtime) != 3) {
+ fclose (fp);
+ goto do_test;
+ }
+ fclose (fp);
+
+ if (generation == MEMO_GENERATION &&
+ (uint64_t) statbuf.st_size == prev_size &&
+ (uint64_t) statbuf.st_mtime == prev_mtime) {
+ /* Same binary as before, so read the previously cached qemu -help
+ * and qemu -devices ? output.
+ */
+ if (access (qemu_help_filename, R_OK) == -1 ||
+ access (qemu_devices_filename, R_OK) == -1)
+ goto do_test;
+
+ debug (g, "loading previously cached test results");
+
+ data = safe_calloc (g, 1, sizeof *data);
+
+ if (guestfs_int_read_whole_file (g, qemu_help_filename,
+ &data->qemu_help, NULL) == -1) {
+ guestfs_int_free_qemu_data (data);
+ return NULL;
+ }
+
+ parse_qemu_version (g, data);
+
+ if (guestfs_int_read_whole_file (g, qemu_devices_filename,
+ &data->qemu_devices, NULL) == -1) {
+ guestfs_int_free_qemu_data (data);
+ return NULL;
+ }
+
+ return data;
+ }
+
+ do_test:
+ data = safe_calloc (g, 1, sizeof *data);
+
+ if (test_qemu (g, data) == -1) {
+ free (data);
+ return NULL;
+ }
+
+ /* Now memoize the qemu output in the cache directory. */
+ debug (g, "saving test results");
+
+ fp = fopen (qemu_help_filename, "w");
+ if (fp == NULL) {
+ help_error:
+ perrorf (g, "%s", qemu_help_filename);
+ if (fp != NULL) fclose (fp);
+ guestfs_int_free_qemu_data (data);
+ return NULL;
+ }
+ if (fprintf (fp, "%s", data->qemu_help) == -1)
+ goto help_error;
+ if (fclose (fp) == -1)
+ goto help_error;
+
+ fp = fopen (qemu_devices_filename, "w");
+ if (fp == NULL) {
+ devices_error:
+ perrorf (g, "%s", qemu_devices_filename);
+ if (fp != NULL) fclose (fp);
+ guestfs_int_free_qemu_data (data);
+ return NULL;
+ }
+ if (fprintf (fp, "%s", data->qemu_devices) == -1)
+ goto devices_error;
+ if (fclose (fp) == -1)
+ goto devices_error;
+
+ /* Write the qemu.stat file last so that its presence indicates that
+ * the qemu.help and qemu.devices files ought to exist.
+ */
+ fp = fopen (qemu_stat_filename, "w");
+ if (fp == NULL) {
+ stat_error:
+ perrorf (g, "%s", qemu_stat_filename);
+ if (fp != NULL) fclose (fp);
+ guestfs_int_free_qemu_data (data);
+ return NULL;
+ }
+ /* The path to qemu is stored for information only, it is not
+ * used when we parse the file.
+ */
+ if (fprintf (fp, "%d %" PRIu64 " %" PRIu64 " %s\n",
+ MEMO_GENERATION,
+ (uint64_t) statbuf.st_size,
+ (uint64_t) statbuf.st_mtime,
+ g->hv) == -1)
+ goto stat_error;
+ if (fclose (fp) == -1)
+ goto stat_error;
+
+ return data;
+}
+
+static int
+test_qemu (guestfs_h *g, struct qemu_data *data)
+{
CLEANUP_CMD_CLOSE struct command *cmd1 = guestfs_int_new_command (g);
CLEANUP_CMD_CLOSE struct command *cmd2 = guestfs_int_new_command (g);
int r;
- struct qemu_data *data;
-
- data = safe_calloc (g, 1, sizeof *data);
guestfs_int_cmd_add_arg (cmd1, g->hv);
guestfs_int_cmd_add_arg (cmd1, "-display");
@@ -107,16 +247,14 @@ guestfs_int_test_qemu (guestfs_h *g)
if (r == -1 || !WIFEXITED (r) || WEXITSTATUS (r) != 0)
goto error;
- return data;
+ return 0;
error:
- free (data);
-
if (r == -1)
- return NULL;
+ return -1;
guestfs_int_external_command_failed (g, r, g->hv, NULL);
- return NULL;
+ return -1;
}
/* Parse the first line of data->qemu_help (if not NULL) into the
--
2.7.4
More information about the Libguestfs
mailing list