On Fri, 11 Mar 2022 05:59:00 GMT, David Holmes <[email protected]> wrote:
> > I ended up changing `os::malloc()` to zero the buffer when running with
> > -Xshare:dump. Hopefully one extra check of `if (DumpSharedSpaces)` doesn't
> > matter too much for regular VM executions because `os::malloc()` already
> > has a high overhead.
>
> This is raising red flags for me sorry. Every user of the JDK is now paying a
> penalty because of something only needed when dumping the shared archive. It
> might not be much but it is the old "death by a thousand cuts". Is there any
> way to tell the OS to pre-zero all memory provided to the current process,
> such that we could set that when dumping and not have to check on each
> allocation?
I don't know how to tell the OS (or C library) to zero out the buffer returned
by malloc. However, in the current code path, we already have a test for an
uncommon condition when `os::malloc()` calls `MemTracker::record_malloc()`
which calls `MallocTracker::record_malloc()`
void* MallocTracker::record_malloc(void* malloc_base, size_t size, MEMFLAGS
flags,
const NativeCallStack& stack)
{
if (MemTracker::tracking_level() == NMT_detail) {
MallocSiteTable::allocation_at(stack, size, &mst_marker, flags);
}
I can combine the tests for `MemTracker::tracking_level()` and
`DumpSharedSpaces` into a single test and do more work only when the uncommon
path is taken. This would require some refactoring of the
MemTracker/MallocTracker code. I'd rather do that in a separate RFE.
In fact, `MemTracker::_tracking_level` is tested twice in the current
implementation. We can change it to do a single test in the most common case
(NMT_summary) if we really want to cut down the number of tests. But honestly I
don't think this makes any difference.
> And I have to wonder how easy it would be to re-introduce non-deterministic
> values in these data structures that are being dumped. Does malloc itself
> even guarantee to return the same set of addresses for the same sequence of
> requests in different executions of a program?
The malloc'ed objects are copied into the CDS archive at deterministic
addresses. Any pointers inside such objects will be relocated.
-------------
PR: https://git.openjdk.java.net/jdk/pull/7748
