On 11/03/2022 4:40 pm, Ioi Lam wrote:
On Fri, 11 Mar 2022 05:59:00 GMT, David Holmes <david.hol...@oracle.com> 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.
Okay. I won't object further but I really don't like it - c'est la vie!
I'll let others review the actual code changes in detail.
Cheers,
David
-------------
PR: https://git.openjdk.java.net/jdk/pull/7748
