<div><p style="-webkit-print-color-adjust: exact; margin-right: 0px; margin-bottom: 15px; margin-left: 0px; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto; margin-top: 0px !important;">Hi, all</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;">We use metaspace instead of perm since JDK8, I found that after the Full GC, the size of the metaspace has always been the same from the GC log. I have looked at the source code of hotspot roughly. It has not bean fixed in the latest version. The problem which may have some impact on us to troubleshoot some metaspace problems.<span class="Apple-converted-space"> </span></p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;">I wrote a test case, and simply analyzed the hotspot code, and showed the output before and after the modification. I hope this problem can be fixed as soon as possible.</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;">Demo：</p><div style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;"><pre style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-bottom: 15px; background-color: rgb(248, 248, 248); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 19px; overflow: auto; padding: 6px 10px; border-radius: 3px;"><code class="language-none" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; white-space: pre; border: none; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-position: initial initial; background-repeat: initial initial;">import java.io.File;
import java.net.URL;
import java.net.URLClassLoader;
/**
*
* @author nijiaben
* @version v1.0
* @createTime 2018年09月22日 12:32:32 PM
*
*/
public class ClassLoaderTest {
private static URL[] urls = new URL[1];
static {
try {
File jarFile = new File("/home/admin/.m2/repository/org/slf4j/slf4j-api/1.7.21/slf4j-api-1.7.21.jar");
urls[0] = jarFile.toURI().toURL();
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String args[]) {
for(int i=0;i<1000;i++) {
loadClass();
}
System.gc();
}
public static void loadClass() {
try {
URLClassLoader ucl = new URLClassLoader(urls);
Class.forName("org.slf4j.Logger", false, ucl);
} catch (Exception e) {
e.printStackTrace();
}
}
}</code></pre></div><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;">My demo is very simple, just constantly create a new class loader to load a specific class, after loading, execute a System GC, so that we can see the relevant GC log output, the JVM parameters we run are</p><div style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;"><pre style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-bottom: 15px; background-color: rgb(248, 248, 248); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 19px; overflow: auto; padding: 6px 10px; border-radius: 3px;"><code class="language-none" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; white-space: pre; border: none; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-position: initial initial; background-repeat: initial initial;">-Xmx1000M -Xms1000M -XX:+UseConcMarkSweepGC -XX:+PrintGCDetails ClassLoaderTest</code></pre></div><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;">The GC log is</p><div style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;"><pre style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-bottom: 15px; background-color: rgb(248, 248, 248); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 19px; overflow: auto; padding: 6px 10px; border-radius: 3px;"><code class="language-none" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; white-space: pre; border: none; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-position: initial initial; background-repeat: initial initial;">[Full GC (System.gc()) [CMS: 0K->9921K(683264K), 0.1003988 secs] 49083K->9921K(989952K), [Metaspace: 12916K->12916K(1064960K)], 0.1060065 secs] [Times: user=0.06 sys=0.05, real=0.11 secs]</code></pre></div><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;">Let's look at the source code of hotspot,</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;"><code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">GenCollectedHeap::do_collection</code>:</p><div style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;"><pre style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-bottom: 15px; background-color: rgb(248, 248, 248); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 19px; overflow: auto; padding: 6px 10px; border-radius: 3px;"><code class="language-none" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; white-space: pre; border: none; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-position: initial initial; background-repeat: initial initial;">if (PrintGCDetails) {
print_heap_change(gch_prev_used);
// Print metaspace info for full GC with PrintGCDetails flag.
if (complete) {
MetaspaceAux::print_metaspace_change(metadata_prev_used);
}
}
for (int j = max_level_collected; j >= 0; j -= 1) {
// Adjust generation sizes.
_gens[j]->compute_new_size();
}
if (complete) {
// Delete metaspaces for unloaded class loaders and clean up loader_data graph
ClassLoaderDataGraph::purge();
MetaspaceAux::verify_metrics();
// Resize the metaspace capacity after full collections
MetaspaceGC::compute_new_size();
update_full_collections_completed();
}</code></pre></div><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;">We see that metaspace's gc log output is executed before<span class="Apple-converted-space"> </span><code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">ClassLoaderDataGraph::purge();<span class="Apple-converted-space"> </span></code>, that means the size of the metaspace is printed without reclaiming the memory of the metaspace, so the GC log seems no change before and after the GC metaspace.</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;">After knowing the specific problem, we can make an adjustment to the code to move the logic of the output of metaspace size after to<span class="Apple-converted-space"> </span><code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">ClassLoaderDataGraph::purge()</code>，just like this:</p><div style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;"><pre style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-bottom: 15px; background-color: rgb(248, 248, 248); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 19px; overflow: auto; padding: 6px 10px; border-radius: 3px;"><code class="language-none" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; white-space: pre; border: none; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-position: initial initial; background-repeat: initial initial;"> if (PrintGCDetails) {
print_heap_change(gch_prev_used);
}
for (int j = max_level_collected; j >= 0; j -= 1) {
// Adjust generation sizes.
_gens[j]->compute_new_size();
}
if (complete) {
// Delete metaspaces for unloaded class loaders and clean up loader_data graph
ClassLoaderDataGraph::purge();
MetaspaceAux::verify_metrics();
// Resize the metaspace capacity after full collections
MetaspaceGC::compute_new_size();
update_full_collections_completed();
if (PrintGCDetails) {
// Print metaspace info for full GC with PrintGCDetails flag.
MetaspaceAux::print_metaspace_change(metadata_prev_used);
}
}</code></pre></div><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;">At this point we recompile hotspot, execute the above demo again, you can see the correct output as follows</p><div style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;"><pre style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-bottom: 15px; background-color: rgb(248, 248, 248); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 19px; overflow: auto; padding: 6px 10px; border-radius: 3px;"><code class="language-none" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; white-space: pre; border: none; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-position: initial initial; background-repeat: initial initial;">[Full GC (System.gc()) [CMS: 0K->9921K(683264K), 0.0852627 secs] 49083K->9921K(989952K), [Metaspace: 12913K->3280K(1058816K)], 0.0891207 secs] [Times: user=0.05 sys=0.04, real=0.09 secs]</code></pre></div><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;">I also probably thought about why this problem is caused. This is the version before JDK8, such as JDK7, you can see the same code location:</p><div style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;"><pre style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-bottom: 15px; background-color: rgb(248, 248, 248); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 19px; overflow: auto; padding: 6px 10px; border-radius: 3px;"><code class="language-none" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; white-space: pre; border: none; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-position: initial initial; background-repeat: initial initial;">if (PrintGCDetails) {
print_heap_change(gch_prev_used);
// Print perm gen info for full GC with PrintGCDetails flag.
if (complete) {
print_perm_heap_change(perm_prev_used);
}
}
for (int j = max_level_collected; j >= 0; j -= 1) {
// Adjust generation sizes.
_gens[j]->compute_new_size();
}
if (complete) {
// Ask the permanent generation to adjust size for full collections
perm()->compute_new_size();
update_full_collections_completed();
}</code></pre></div><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;">It can be seen that the GC engineer at that time simply replaced<span class="Apple-converted-space"> </span><code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">print_perm_heap_change(perm_prev_used);</code>with<span class="Apple-converted-space"> </span><code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">MetaspaceAux::print_metaspace_change(metadata_prev_used);</code>but forgot the difference between Perm and Metaspace.</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto;">Thanks,</p><p style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-right: 0px; margin-left: 0px; font-family: Helvetica, arial, sans-serif; text-size-adjust: auto; margin-bottom: 0px !important;">nijiaben</p></div><div><includetail><!--<![endif]--></includetail></div>