> This is a continuation of https://github.com/openjdk/jdk/pull/10085. I closed > https://github.com/openjdk/jdk/pull/10085 because it had accumulated too much > comment history and got confusing. For a history of this issue, see previous > discussions [1] and the comment section of 10085. > > --------------- > > This RFE adds the option to trim the Glibc heap periodically. This can > recover a significant memory footprint if the VM process suffers from > high-but-rare malloc spikes. It does not matter who causes the spikes: the > JDK or customer code running in the JVM process. > > ### Background: > > The Glibc is reluctant to return memory to the OS. Temporary malloc spikes > often carry over as permanent RSS increase. Note that C-heap retention is > difficult to observe. Since it is freed memory, it won't appear in NMT; it is > just a part of RSS. > > This is, effectively, caching - a performance tradeoff by the glibc. It makes > a lot of sense with applications that cause high traffic on the C-heap. The > JVM, however, clusters allocations and often rolls its own memory management > based on virtual memory for many of its use cases. > > To manually trim the C-heap, Glibc exposes `malloc_trim(3)`. With JDK 18 [2], > we added a new jcmd command to *manually* trim the C-heap on Linux (`jcmd > System.trim_native_heap`). We then observed customers running this command > periodically to slim down process sizes of container-bound jvms. That is > cumbersome, and the JVM can do this a lot better - among other things because > it knows best when *not* to trim. > > #### GLIBC internals > > The following information I took from the glibc source code and experimenting. > > ##### Why do we need to trim manually? Does the Glibc not trim on free? > > Upon `free()`, glibc may return memory to the OS if: > - the returned block was mmap'ed > - the returned block was not added to tcache or to fastbins > - the returned block, possibly merged with its two immediate neighbors, had > they been free, is larger than FASTBIN_CONSOLIDATION_THRESHOLD (64K) - in > that case: > a) for the main arena, glibc attempts to lower the brk() > b) for mmap-ed heaps, glibc attempts to completely unmap or shrink the heap. > In both cases, (a) and (b), only the top portion of the heap is reclaimed. > "Holes" in the middle of other in-use chunks are not reclaimed. > > So: glibc *may* automatically reclaim memory. In normal configurations, with > typical C-heap allocation granularity, it is unlikely. > > To increase the chance of auto-reclamation happening, one can do one or more > t...
Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision: Bikeshed Trim log lines ------------- Changes: - all: https://git.openjdk.org/jdk/pull/14781/files - new: https://git.openjdk.org/jdk/pull/14781/files/e821d518..c8d46436 Webrevs: - full: https://webrevs.openjdk.org/?repo=jdk&pr=14781&range=09 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14781&range=08-09 Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod Patch: https://git.openjdk.org/jdk/pull/14781.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/14781/head:pull/14781 PR: https://git.openjdk.org/jdk/pull/14781
