On Wed, 27 Jan 2021 19:21:35 GMT, Chris Plummer <cjplum...@openjdk.org> wrote:
>> Dear All, >> I have made a draft patch on parallel heap dump of jmap. >> This patch is still WIP with test case under development and code refining >> is in process. >> >> But I want to firstly ask your help to discuss a potential compatibility >> issue that may need to solve: >> >> This patch introduces a new "parallel" option to jmap, so after this patch, >> there can be at most 4 options for jmap that can be passed to JDK. >> >> The issue is that there is a limitation in old version of jmap that at most >> 3 arguments can be accepted. As I commented in File JMap.java. and I can not >> simply enlarge the limitation to 4 because that will cause old version of >> jmap hang when working on new JDK. >> >> So I use the 3rd argument of jmap to be a combined string and parse it to >> JDK, and the attachListener.cpp is modified to add the logic for parsing it >> to get gzlevel and parallel thread number. >> >> The current status is that, with current change in this patch, old jmap >> could work normally on new JDK, and new jmap without "parallel" could work >> correctly with old JDK. >> >> But the problem comes when new jmap use "gz=1, parallel=2" options together >> to communicate to an old JDK, the "gz" option takes no effect. The root >> cause is that old JDK's attachListener.cpp does not have the parsing logic, >> so it treat "gz=1, parallel=2" as a whole argument, and hence can not >> recognize it. >> >> May I ask your suggestion to see whether this is behavior is acceptable, and >> if not, any advice for fixing the compatibility issue? >> >> Thanks! >> Lin > >> This patch introduces a new "parallel" option to jmap, so after this patch, >> there can be at most 4 options for jmap that can be passed to JDK. >> >> The issue is that there is a limitation in old version of jmap that at most >> 3 arguments can be accepted. As I commented in File JMap.java. and I can not >> simply enlarge the limitation to 4 because that will cause old version of >> jmap hang when working on new JDK. > > A year or so ago we ran into this same issue. I'll see if I can find the CR > for it. It looks like [JDK-8215622](https://bugs.openjdk.java.net/browse/JDK-8215622) introduced a 4th argument to the attach API, and this caused hangs when and older JVMs tried to attach. That was fixed by [JDK-8219721](https://bugs.openjdk.java.net/browse/JDK-8219721), and seemed to revert the arg count back to 3, although it's not too clear to me how this was accomplished. The webrev is here: http://cr.openjdk.java.net/~xiaofeya/8219721/webrev.01/ Also at the same time the following was filed, which has not yet been addressed: [JDK-8219896](https://bugs.openjdk.java.net/browse/JDK-8219896) Parameter size mismatch between client and VM sides of the Attach API @linzang JDK-8215622 and JDK-8219721 were your CRs, so maybe you can elaborate on them a bit more. ------------- PR: https://git.openjdk.java.net/jdk/pull/2261
