Re: [libvirt] [PATCH v2 3/3] Cache result of QEMU capabilities extraction
On 03/10/2014 10:54 AM, Daniel P. Berrange wrote:
Extracting capabilities from QEMU takes a notable amount of time
when all QEMU binaries are installed. Each system emulator
needs about 200-300ms multiplied by 26 binaries == ~5-8 seconds.
This change causes the QEMU driver to save an XML file containing
the content of the virQEMUCaps object instance in the cache
dir eg /var/cache/libvirt/qemu/capabilities/$SHA256(binarypath).xml
or $HOME/.cache/libvirt/qemu/cache/capabilities/$SHA256(binarypath).xml
Good, http://www.tldp.org/LDP/Linux-Filesystem-Hierarchy/html/var.html
states that /var/cache normally persists across reboots.
We attempt to load this and only if it fails, do we fallback to
probing the QEMU binary. The ctime of the QEMU binary and libvirtd
are stored in the cached file and its data discarded if either
of them change.
Signed-off-by: Daniel P. Berrange <berrange(a)redhat.com>
---
src/qemu/qemu_capabilities.c | 436 ++++++++++++++++++++++++++++++++++++++++++-
src/qemu/qemu_capabilities.h | 2 +
src/qemu/qemu_driver.c | 1 +
3 files changed, 431 insertions(+), 8 deletions(-)
+
+/*
+ * Update the XML parser/formatter when adding more
+ * information to this struct so that it gets cached
+ * correctly. It does not have to be ABI-stable. A
+ * newer libvirtd will simply discard the cache if
+ * it fails to parse the XML doc and save a new one.
A newer libvirtd will discard the cache anyways, because the timestamp
differs from the cached ctime.
+/*
+ * Parsing a doc that looks like
+ *
+ * <qemuCaps>
+ * <usedQMP/>
+ * <flag name='foo'/>
+ * <flag name='bar'/>
+ * ...
+ * <cpu name="pentium3"/>
+ * ...
+ * <machine name="pc-1.0" alias="pc"
maxCpus="4"/>
+ * ...
+ * </qemuCaps>
Ought to mention the cached ctime elements.
+ */
+static int
+virQEMUCapsLoadCache(virQEMUCapsPtr qemuCaps, const char *filename,
+ time_t *qemuctime, time_t *selfctime)
+{
+ if (virXPathLongLong("string(./qemuctime)", ctxt,
&l) < 0) {
+ virReportError(VIR_ERR_XML_ERROR, "%s",
+ _("missing qemuctime in QEMU capabilities XML"));
+ qemuCaps->usedQMP = virXPathBoolean("count(.//usedQMP)
> 0",
Why ./qemuctime but .//usedQMP? What difference does // make in XPath?
+
+static int
+virQEMUCapsSaveCache(virQEMUCapsPtr qemuCaps, const char *filename)
+{
+ virBuffer buf = VIR_BUFFER_INITIALIZER;
+ const char *xml = NULL;
+ int ret = -1;
+ size_t i;
+
+ virBufferAddLit(&buf, "<qemuCaps>\n");
+
+ virBufferAsprintf(&buf, " <qemuctime>%llu</qemuctime>\n",
Conflicts with Laine's work to require virBufferAddIndent() rather than
hard-coding indentation. Will be interesting to see who gets in first :)
+static void
+virQEMUCapsReset(virQEMUCapsPtr qemuCaps)
+{
+ size_t i;
+
+ virBitmapClearAll(qemuCaps->flags);
+ qemuCaps->version = qemuCaps->kvmVersion = 0;
+ qemuCaps->arch = VIR_ARCH_NONE;
Shouldn't you also reset qemuCaps->usedQMP and qemuCaps->ctime, to get
the struct back to a known-default state? Or is this only ever going to
be used just before freeing the struct, at which point all the
reset-to-0 code is wasted CPU cycles?
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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