On Mon, 29 Aug 2022 20:13:46 GMT, Chris Plummer <cjplum...@openjdk.org> wrote:
> 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. This pull request has now been integrated. Changeset: c3380c03 Author: Chris Plummer <cjplum...@openjdk.org> URL: https://git.openjdk.org/jdk/commit/c3380c03b12fcf9e8fd146525ff6ffb6e3c286bf Stats: 48 lines in 4 files changed: 0 ins; 40 del; 8 mod 8292995: improve the SA page cache Reviewed-by: kevinw, sspitsyn ------------- PR: https://git.openjdk.org/jdk/pull/10069
