RFR: 8241825: Make compressed oops and compressed class pointers independent on x86_64
Erik Österlund
erik.osterlund at oracle.com
Tue Apr 28 16:28:21 UTC 2020
Hi Thomas,
Thanks for reviewing this.
On 2020-04-28 13:14, Thomas Stüfe wrote:
> Hi Erik,
>
> On Tue, Apr 28, 2020 at 10:07 AM Erik Österlund
> <erik.osterlund at oracle.com <mailto: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.
Right, so the purpose of this flag was to be able to move the heap up,
because if we map things too low, we ran into conflicts with sbrk, and
hence some old suboptimal malloc implementations, such as the default
malloc on Solaris. This called for a flag to be able to move the heap
up, putting it out of the way from sbrk. That is how HeapBaseMinAddress
came about, AFAIK. The situation when mapping in metaspace and
compressed class space, without compressed oops, is very much the same:
we want to go as low as possible for optimal encoding, but not too low
so we run into these problems.
> 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...).
Right. And yes I'm pretty sure we do check. The heap reserves memory,
and sets the CompressedOops::end() based on that, which is fed as input
to the metaspace/CDS placement.
> 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?
Exactly.
> 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.
Yes agreed. Ideally it would be called something like MappingMinAddress
instead, because it really isn't tied to the heap really, it just
happened to be the primary user of mapping things at low addresses. But
I also think changing that, and enforcing it more widely in the os layer
sounds like a different 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.
I think it looks like I do that.
> - If HeapBaseMinAddress is now always used, we should check it always
> too (see Arguments::set_heap_size)
Good idea. Fixed.
>
>> 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.
Yup. I added some explicit test cases for -XX:-UseCompressedOops
-XX:+UseCompressedClassPointers as suggested. This will also be the
default configuration for ZGC, so that should give that configuration
more test coverage in the various tests we have.
Here is a new webrev:
http://cr.openjdk.java.net/~eosterlund/8241825/webrev.01/
Incremental:
http://cr.openjdk.java.net/~eosterlund/8241825/webrev.00_01/
I hope I fixed everything you wanted in this patch. I also fixed what
Frederic and Coleen wanted.
Thanks,
/Erik
> 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 <mailto: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
>>
>
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