The page caching support in SA is woefully dated. I think it has stayed the
same for over 20 years when it was originally done for solarix-x86. This code
has been replicated for every port. Currently all ports only have a 16mb cache.
They use 4k pages and there are 4k of them.
I think the 4k page size is fine. The following comment appears in all the
ports:
// ... This is a cache of 4096 4K pages, or 16 MB. The page
// size must be adjusted to be the hardware's page size.
// (FIXME: should pick this up from the debugger.)
I disagree with this. Matching the possibly very large hardware page size (I
think maybe they meant OS page size) would require the SA page cache to be very
very large, using a lot of java heap space. It would also require a lot of
unnecessary copying from the debuggee process's memory. There's no reason for
the SA cache's page size to match the OS page size.
However, 16mb seems very small. I tried 256mb and this gave about a 10%
performance improvement in a heap dump, and is still fairly small, so I think
it is a reasonable adjustment.
Another comment you see in all the ports (copied from solaris-x86) is:
// FIXME: re-test necessity of cache on Linux, where data
// fetching is faster
// Cache portion of the remote process's address space.
// Fetching data over the socket connection to dbx is slow.
// Might be faster if we were using a binary protocol to talk to
// dbx, but would have to test. For now, this cache works best
// if it covers the entire heap of the remote process. FIXME: at
At least on linux the cache is definitely needed. I turned it off and a heap
dump took 9x longer. Also I think covering the entire heap is overkill, and I
doubt was ever being done given how small the cache is. So I think this comment
can just be removed.
-------------
Commit messages:
- Do some tuning and cleanup of SA page cache.
Changes: https://git.openjdk.org/jdk/pull/10069/files
Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10069&range=00
Issue: https://bugs.openjdk.org/browse/JDK-8292995
Stats: 48 lines in 4 files changed: 0 ins; 40 del; 8 mod
Patch: https://git.openjdk.org/jdk/pull/10069.diff
Fetch: git fetch https://git.openjdk.org/jdk pull/10069/head:pull/10069
PR: https://git.openjdk.org/jdk/pull/10069
