RFR(S) 8218751 Do not store original classfiles inside the CDS archive
Ioi Lam
ioi.lam at oracle.com
Sat Feb 16 02:56:35 UTC 2019
On 2/15/19 4:54 PM, Jiangli Zhou wrote:
> Hi,
>
> To answer your use case question, one of the case reported last year
> in OpenJDK was JRebel (please go back to the hotspot-dev mail list
> 2018 Oct. archive). That is an existing example as I've tried to point
> out in my earlier email.
>
First of all, if you read the email carefully, CDS was not even in their
usual test matrix. Also, if you're modifying classes on the fly, you're
slowing down start-up big time. A few ms spent in decoding the classfile
data will be completely drown out by the overhead of the classfile
parsing and patching code in the JVMTI agent.
So no, JRebel will not start up a few % faster if CDS stores the
classfile data. It will most likely be not measurable.
> The issue with the current change is that it's only targeted for
> reducing the static footprint (static footprint reduction could be
> achieved with alternative approaches such as file compression). The
> removal of the 'od' space and the benefits (both the startup and
> runtime footprint) is not backed up by a clear requirement here and
> the change should not go in as is.
>
As part of the JVM development, we constantly review and remove
unnecessary code to keep the JVM healthy. That's a clear requirement and
mandate on Oracle as the major contributor to the JDK.
In this case, we are removing a premature optimization in the original
design. The classfile data stored in the CDS archive are rarely used
(99.99+%), and when they are used, the speed optimization is irrelevant
because the user of this data (JVMTI agent) is so much slower. So
everyone is taking a footprint hit with no benefit to anyone.
Thanks
- Ioi
> Just to summarize the drawbacks of the current change:
>
> * Runtime footprint increase (causes more memory fragmentation even
> the memory is freed after use)
> * Startup time regression when can_generate_all_class_hook_events
> capability is enabled
> * May cause issue with future optimization
>
> Thanks,
> Jiangli
>
>
>
>
>
> On Fri, Feb 15, 2019 at 2:07 PM Ioi Lam <ioi.lam at oracle.com
> <mailto:ioi.lam at oracle.com>> wrote:
>
> Hi Jiangli,
>
> Thanks for volunteering on this. I think it's best to do that
> (optional support for storing uncompressed classfile data into the
> CDS archive) as a separate bug than the current issue.
>
> If you decide to provide a patch for that, I think it would be
> best to find actual users or use cases that would find it
> beneficial. Otherwise it will appear to be a solution looking for
> a problem.
>
> Thanks
>
> - Ioi
>
>
> On 2/15/19 10:35 AM, Jiangli Zhou wrote:
>> I'm willing to continue contributing to the support and
>> maintaining of CDS/AppCDS. Often startup improvements with
>> measurable gain (>4% in this case) are non-trivial. The support
>> for 'od' is not overly complex comparing to the performance gain.
>> In a rosier picture if Jlink/AOT/CDS can work together in harmony
>> to create a single image in the future, class files (and even JAR
>> files) can be eliminated and no class file data can be loaded at
>> runtime from a JAR file. We should consider all aspects and not
>> make a decision lightly.
>>
>> For the dynamic archiving, this is not a blocking issue. I can
>> step in an help with making 'od' optional if needed.
>>
>> Thanks,
>> Jiangli
>>
>> On Thu, Feb 14, 2019 at 7:53 PM Ioi Lam <ioi.lam at oracle.com
>> <mailto:ioi.lam at oracle.com>> wrote:
>>
>> That means we have to add a new -XX option to enable this,
>> and need to add extra test cases and maintenance.
>>
>> We are talking about only single digit % benefits in very
>> narrow use cases, and it's not clear whether performance is
>> important in such cases (or if people would bother to use
>> this -XX flag to gain a few %). Without user input, it's hard
>> to justify keep spending time on an optimization that
>> literally no one had asked for.
>>
>> I just don't think we have time to keep maintaining this. If
>> someone is willing to step up and provide support for this,
>> feel free.
>>
>> Thanks
>>
>> - Ioi
>>
>>
>> On 2/14/19 1:20 PM, Jiangli Zhou wrote:
>>> Based on the issues raised in OpenJDK last year, there
>>> appears to be existing use cases with AppCDS + CLFH. When
>>> designing the platform, we should think from the user
>>> perspective. Making 'od' optional is not complex and gives
>>> the control to the Java users. It also avoids potential
>>> waste of effort for removing&putting back the optimization.
>>>
>>> Thanks,
>>> Jiangli
>>> On Thu, Feb 14, 2019 at 11:46 AM Ioi Lam <ioi.lam at oracle.com
>>> <mailto:ioi.lam at oracle.com>> wrote:
>>>
>>> Such optional support increase the complexity of the VM.
>>> My preference would be to wait until there's an actual
>>> need for this.
>>>
>>> The JDK is release under 6 months cadence, so if the
>>> removal turns out to be a bad idea, we can reinstate the
>>> code in the next release. Or, someone can just make
>>> their own JDK build with a simple anti-delta of this patch.
>>>
>>> If we are too reluctant to remove anything, the JDK will
>>> eventually become a hopeless mess.
>>>
>>> Thanks
>>>
>>> - Ioi
>>>
>>> On 2/14/19 10:43 AM, Jiangli Zhou wrote:
>>>> Sorry for the slow turnaround. Hopefully it will get
>>>> better after this week. As there is no enough user
>>>> data/requirement to determine which optimization
>>>> direction is more important in this case, it might be
>>>> reasonable to make the 'optional data' space truly
>>>> optional, which can be controlled by a command-line
>>>> option. If the performance is more important (in case
>>>> should_post_class_file_load_hook is required for a
>>>> particular use case), user can enable dumping out the
>>>> 'od' space with archived class file data, otherwise the
>>>> data can be loaded at runtime. What's your thought on this?
>>>>
>>>> Thanks,
>>>> Jiangli
>>>>
>>>> On Wed, Feb 13, 2019 at 12:03 PM Ioi Lam
>>>> <ioi.lam at oracle.com <mailto:ioi.lam at oracle.com>> wrote:
>>>>
>>>>
>>>> On 2/13/19 11:24 AM, Jiangli Zhou wrote:
>>>>> Hi Ioi,
>>>>>
>>>>> Thanks for the additional information and
>>>>> performance data. Please see more comments below.
>>>>>
>>>>> On Wed, Feb 13, 2019 at 5:33 AM Ioi Lam
>>>>> <ioi.lam at oracle.com <mailto:ioi.lam at oracle.com>>
>>>>> wrote:
>>>>>
>>>>> Hi Jiangli,
>>>>>
>>>>> The main reason for doing this is to reduce
>>>>> the size of the CDS file.
>>>>> For example, when archiving Eclipse IDE with
>>>>> JDK-8215311 (Dynamic Class
>>>>> Metadata Archive), the CDS archive is reduced
>>>>> from 100MB to about 67MB.
>>>>>
>>>>> We have not heard any requirement for high
>>>>> performance with
>>>>> CDS+ClassFileLoadHook. It doesn't seem right
>>>>> for everyone to take a 50%
>>>>> file size penalty for an optimization that no
>>>>> one has asked for.
>>>>>
>>>>>
>>>>> Jut to clarify, it is ~30% not 50% (possibly a
>>>>> typo?), correct?
>>>>
>>>>
>>>> Depending on how you look at it. Going from 67MB to
>>>> 100MB is an increase of 33MB which is 50%.
>>>>
>>>>
>>>>>
>>>>> Here are some perf numbers (running "java
>>>>> -version" with an agent that
>>>>> installs a CFLH that doesn't nothing). Yes,
>>>>> it's somewhat slower as
>>>>> expected, but it doesn't seem to be
>>>>> catastrophic. Using CDS is
>>>>> nevertheless much faster than without CDS anyway.
>>>>>
>>>>> After No CFLH 0.0476 seconds time elapsed (
>>>>> +- 0.20% )
>>>>> After with CFLH 0.0513 seconds time elapsed (
>>>>> +- 0.18% ) 7.773%
>>>>> slower
>>>>>
>>>>> Before no CFLH 0.0472 seconds time elapsed (
>>>>> +- 0.21% )
>>>>> Before with CFLH 0.0492 seconds time elapsed
>>>>> ( +- 0.18% ) 4.237%
>>>>> slower
>>>>>
>>>>>
>>>>> Comparing 'After with CFLH' (0.0513 seconds) with
>>>>> 'Before with CFLH' (0.0492 seconds), there is
>>>>> about 4.27% performance degradation observed from
>>>>> your data.
>>>>>
>>>>> Another disadvantage of this proposed change is
>>>>> the increasing of runtime memory. When the class
>>>>> file data is stored in the shared archive file and
>>>>> mapped as RO (read-only), it can be shared among
>>>>> multiple JVM processes. For the default CDS
>>>>> archive case, it's shared by all JVM processes
>>>>> running simultaneously. With the proposed change
>>>>> in the webrev, the memory for the class file data
>>>>> is allocated at runtime. It moves the cost from
>>>>> static footprint to runtime memory footprint,
>>>>> which is a higher price because it needs to be
>>>>> multiplied by the number of running JVM processes.
>>>>
>>>>
>>>> With this patch, the decoded classdata is freed
>>>> after the CFLH is called, so there's no resident
>>>> memory cost.
>>>>
>>>>
>>>>>
>>>>>
>>>>> CDS Off with CFLH 0.0869 seconds time elapsed
>>>>> ( +- 0.18% )
>>>>> CDS Off without CFLH 0.0852 seconds time
>>>>> elapsed ( +- 0.14% ) 1.995%
>>>>> slower
>>>>>
>>>>> I am not sure if there's a performance
>>>>> critical case for CFLH.
>>>>>
>>>>>
>>>>> I have the similar question. It would be good to
>>>>> go back and re-investigate the original
>>>>> requirements for supporting CFLH use cases.
>>>>
>>>>
>>>> I think when we added CFLH support for CDS, the
>>>> goal was "to not lose all benefit of CDS when CFLH
>>>> is enabled". I think we still achieve that after
>>>> this patch. There really was no requirement "to
>>>> make CFLH blazing fast at all cost".
>>>>
>>>> Thanks
>>>>
>>>> - Ioi
>>>>
>>>>> I suppose
>>>>> if performance is critical you would rewrite
>>>>> the classes statically and
>>>>> rebuild your app, instead of patching its
>>>>> bytecodes at runtime. However,
>>>>> if there were indeed a performance critical
>>>>> case, I think it's better to
>>>>> change JVMTI to allow a 2-level filtering for
>>>>> CFLH:
>>>>>
>>>>> I believe the overwhelming use case, where
>>>>> performance is critical, the
>>>>> CFLH will just patch a small number of class
>>>>> files. I can't fathom any
>>>>> use case where someone wants to patch EVERY
>>>>> loaded class.
>>>>>
>>>>> The current CFLH passes both the name of the
>>>>> class, as well as the
>>>>> classfile data, in a single call. This forces
>>>>> CDS to decode the
>>>>> classfile data for every hook call. However,
>>>>> in most cases, the CFLH
>>>>> will just examine the class name, and do
>>>>> nothing unless the name matches
>>>>> a certain pattern, so we end up wasting the
>>>>> decoding effort.
>>>>>
>>>>> My suggested improvement is to add a new
>>>>> filtering call in JVMTI that
>>>>> passes only the name. If the CFLH wants to
>>>>> patch the class, it will then
>>>>> request the classfile data, at which point CDS
>>>>> will decode it from the
>>>>> modules file or JAR file.
>>>>>
>>>>>
>>>>> That probably is the right direction.
>>>>>
>>>>> Thanks,
>>>>> Jiangli
>>>>>
>>>>>
>>>>> Thanks
>>>>>
>>>>> - Ioi
>>>>>
>>>>>
>>>>> On 2/12/19 5:18 PM, Jiangli Zhou wrote:
>>>>> > Hi Ioi,
>>>>> >
>>>>> > I'd like to understand the performance
>>>>> impact with this change. Do you have
>>>>> > any performance numbers when
>>>>> JvmtiExport::should_post_class_file_load_hook()
>>>>> > is required? This is a performance vs
>>>>> footprint trade-off. For some users,
>>>>> > performance is more important than static
>>>>> footprint.
>>>>> >
>>>>> > Could you also please provide some
>>>>> background/motivation for this change?
>>>>> >
>>>>> > Thanks,
>>>>> > Jiangli
>>>>> >
>>>>> > On Mon, Feb 11, 2019 at 9:24 AM Ioi Lam
>>>>> <ioi.lam at oracle.com
>>>>> <mailto:ioi.lam at oracle.com>> wrote:
>>>>> >
>>>>> >>
>>>>> http://cr.openjdk.java.net/~iklam/jdk13/8218751-dont-store-classfiles-in-cds.v01/
>>>>> >>
>>>>> https://bugs.openjdk.java.net/browse/JDK-8218751
>>>>> >>
>>>>> >> For JVMTI ClassFileLoadHook support, the
>>>>> CDS archive currently stores
>>>>> >> the original classfile data of all archived
>>>>> classes.
>>>>> >>
>>>>> >> However, this consists of over 30% of the
>>>>> archive size. Because all
>>>>> >> original classfile data are already
>>>>> available in other files (such as the
>>>>> >> JDK lib/modules file, or JAR files in the
>>>>> classpath), we can simply read
>>>>> >> from these locations when needed by JVMTI.
>>>>> >>
>>>>> >> For the default CDS archive (included as
>>>>> part of the JDK distribution),
>>>>> >> the size is reduced from about 18.5MB to
>>>>> 12.1MB on Linux/x64.
>>>>> >>
>>>>> >> Thanks
>>>>> >>
>>>>> >> - Ioi
>>>>> >>
>>>>> >>
>>>>>
More information about the hotspot-runtime-dev
mailing list