RFR: 8241825: Make compressed oops and compressed class pointers independent on x86_64

[Thomas Stüfe]

Hi Erik,

On Tue, Apr 28, 2020 at 10:07 AM Erik Österlund <erik.osterlund at oracle.com>
wrote:

> Hi Thomas,
>
> On 2020-04-28 08:43, Thomas Stüfe wrote:
>
> Hi Erik,
>
> nice change, and now I do not have to keep in mind "heap>32G = no class
> space" :)
>
>
> Glad you liked it!
>
> I eyeballed the metaspace changes (what little there are).
>
> This:
>
> -     char* base = (char*)align_up(CompressedOops::end(), _reserve_alignment);+     char* base;+     if (UseCompressedOops) {+       base = (char*)align_up(CompressedOops::end(), _reserve_alignment);+     } else {+       base = (char*)HeapBaseMinAddress;+     }
>
> may not work as intended. You probably want the end of the reserved heap as attach point for ccs, not the HeapBaseMinAddress.
>
>
> Could you please expand on that? To make it clear, what this code does is
> to put metaspace at the
> minimum address we are allowed to map, when compressed oops is disabled.
> The reasoning for doing
> that is that if the low address bits (resulting in good compressed
> pointers) are not used by compressed oops,
> then they are available to be used by compressed class pointers instead,
> such that at least compressed class
> pointers can utilize this VA to get optimal encoding of class pointers.
>
> Is there anything not desirable about this? If so, I can't see it, and
> would appreciate if you expand on why
> that would be a bad idea. Perhaps there is something I do not see here.
>
>
Oh, this was a long standing confusion in my head. I never noticed that
HeapBaseMinAddress exists for the sole purpose of compressed oops, and is
ignored otherwise. So I thought what you do there is just mapping ccs over
the start of where the user wants the heap to start, and since that may not
work we would fall back to map-anywhere.

So you are saying HeapBaseMinAddress is the lowest address the platform
allows us to map to, irregardless of whether we run with compressed oops or
not.

With compressed oops we continue to squeeze ccs in after the heap in the
hope that it all still fits into the lower 4G or 32G. (Do we ever bother to
check if heap is actually reserved in low area? It may have given up and
reserved upstate instead, leaving free room below...).

Now with your patch without compressed oops we attempt to map at
HeapBaseMinAddress since this is the lowest address we are allowed to map
at and the heap won't obviously need it, right?

My only misgiving would be that HeapBaseMinAddress is even more of a
misnomer (its already now a bit, witness my year long confusion :-), since
it now controls not only heap placement but ccs too. Actually if it is that
important to avoid mapping to addresses below HeapBaseMinAddress we should
probably prevent it somewhere below in the os layer but I really do not
want to open that can of worms.

--

Some more remarks about arguments.cpp:

- In Arguments::set_use_compressed_klass_ptrs(), if now compressed class
pointers are independent from compressed oops, checking
CompressedClassSpaceSize for validity should now always happen for
UseCompressedClassPointers=1.
- If HeapBaseMinAddress is now always used, we should check it always too
(see Arguments::set_heap_size)

Also, could we have some tests which exercise cds/metaspace initialization
> without coops and with ccps? Easiest way would be to extend
> "CompressedOops/CompressedClassPointers.java" by some cases.
>
>
>
Thank you. I think this makes sense especially since compressed class
pointer encoding is restricted to one mode if CDS is on (base = shared
address and shift=3), and since CDS=on is the default zerobased encoding
may not happen that often.


> I will have a look at that for the next webrev.
>
> Thanks for the review.
>
> /Erik
>
>

Thanks,

Thomas



> Thank you, Thomas
>
>
> On Fri, Apr 24, 2020 at 10:19 AM Erik Österlund <erik.osterlund at oracle.com>
> wrote:
>
>> Hi,
>>
>> Today, the use of compressed class pointers assumes the use of
>> compressed oops.
>> This patch loosens up the relationship between compressed oops and
>> compressed
>> class pointers, so that compressed class pointers can be used without
>> compressed
>> oops. This benefits for example ZGC that wants compressed class
>> pointers, but
>> not compressed oops, effectively giving ZGC 4 bytes smaller objects.
>>
>> Much of the complexity of the change is that r12 used to be used by
>> compressed
>> class pointers as some kind of semi-fast temp register that one would
>> restore
>> to the compressed oops heap base (or zero). That made it effectively a
>> slightly
>> optimized spilling mechanism used to find a temp register. I replaced that
>> mechanism with a plain old normal temp register that you pass in as a
>> parameter.
>> The bad news is that I had to find available temp registers in a lot of
>> places.
>> The good news is that almost always were there temp registers available
>> for free,
>> and hence the new temp register is even faster than the old optimized
>> spilling
>> mechanism. Because we almost never need any spilling at all in the
>> contexts where
>> this is used.
>>
>> Since I want the 4 new bytes to actually make objects smaller, I poked
>> the new
>> layout code a bit so that the klass gap is made available for fields.
>> That used
>> to be made available only with compressed oops enabled, due to
>> restrictions in
>> the layout code. Our new layout code does not have such restrictions, and
>> so
>> I will make the 4 bytes available for fields when the new layout code is
>> used
>> and compressed class pointers are used.
>>
>> Now I'm only fixing this for HotSpot x86_64. Ideally the use of
>> compressed oops
>> and compressed class pointers should not be entangled in other platforms
>> and
>> Graal. But that is beyond the scope of this change, and I will let them
>> behave
>> the way that they used to, to be potentially fixed later.
>>
>> Bug:
>> https://bugs.openjdk.java.net/browse/JDK-8241825
>>
>> Webrev:
>> http://cr.openjdk.java.net/~eosterlund/8241825/webrev.00/
>>
>> Testing:
>> hs-tier1-7
>>
>> Thanks,
>> /Erik
>>
>
>