Re: [Libguestfs] [PATCH 4/4] lib: qemu: Memoize qemu feature detection.

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);