This thread brings up the question ... when are we going to switch to
JDK 9 as the required boot JDK? Is there any compelling reason to -not-
upgrade yet?
-- Jon
On 10/3/17 1:37 PM, Magnus Ihse Bursie wrote:
It might be the case that we should not really continue with this as
long as we have JDK 8 as default boot jdk.
If so, we should at least add a -Xshare:auto, which will allow the
user to manually add a system-wide CDS archive to their boot jdk, and
benefit from it.
And then we'll revisit this when we switch to JDK 9, to see if
"-XX:-VerifySharedSpaces -XX:SharedArchiveFile=x" is worth the effort.
Sounds reasonable?
/Magnus
On 2017-10-03 16:45, Erik Joelsson wrote:
The change looks good, but unfortunately I don't see a lot of gain.
First of all it seems the -XX:SharedArchiveFile option removes a lot
of the performance gain. Here are some rough numbers from my machine.
All of them are "time make images" with a clean build directory, on a
16 core/32 threads linux box. In some cases I did two runs, which
further shows the amount of variance in the build time.
Baseline: boot jdk 8, no cds
real 3m14.688s
user 38m34.132s
sys 5m4.032s
real 3m11.086s
user 38m16.820s
sys 4m35.344s
Magnus second patch: boot jdk 8, cds SharedArchiveFile
real 3m6.268s
user 38m9.168s
sys 4m13.392s
real 3m15.985s
user 37m55.328s
sys 3m50.140s
Magnus first patch: boot jdk 8, cds in classes.jsa
real 2m55.972s
user 37m59.636s
sys 4m1.020s
Baseline with boot jdk 9, no cds
real 3m14.262s
user 40m9.216s
sys 5m5.480s
Magnus first patch: boot jdk 9, cds in classes.jsa
real 3m12.721s
user 39m29.332s
sys 4m25.472s
Magnus second patch: boot jdk 9, -XX:SharedArchiveFile,
-XX:-VerifySharedSpaces
real 3m4.297s
user 39m24.556s
sys 4m31.980s
So from this (certainly limited) data set, the only configuration
that shows meaningful improvement is with boot jdk 8 and the first
patch, where we stealth add classes.jsa to the boot jdk. I find this
a very bad thing to do in principle so would vote against such a
solution anyway. Also note that we will switch default boot from 8 to
9 very soon, so it's really the results on 9 that are relevant here.
In 9, there is another option, -XX:-VerifySharedSpaces which removes
most of the overhead added by -XX:SharedArchiveFile. Perhaps using
that this becomes worth while.
I would like to see more data showing the optimization is actually
relevant. On my machine it seems to make a measurable difference, but
not a very significant one.
One note on the proposed change. While working on this, when I
changed my boot jdk, configure did not regenerate the classes.jsa
file. I believe for safety we should regenerate on every configure
run. It's not that expensive anyway.
/Erik
On 2017-10-03 14:55, David Holmes wrote:
Looks good to me.
Thanks,
David
On 3/10/2017 10:47 PM, Magnus Ihse Bursie wrote:
On 2017-10-03 14:21, David Holmes wrote:
Erik J. raises a good point in the bug report that
-XX:SharedArchiveFile=xxx should be used if we create the archive.
The build system has no business creating an archive inside the
boot JDK.
Agree, that is a better solution. I was not aware of the
-XX:SharedArchiveFile option.
Here's an updated webrev:
http://cr.openjdk.java.net/~ihse/JDK-8188312-use-CDS-for-bootjdk/webrev.02
I create the jsa file in configure-support, that way it can survive
a "make clean".
/Magnus
David
On 3/10/2017 9:02 PM, David Holmes wrote:
Hi Claes,
On 3/10/2017 8:48 PM, Claes Redestad wrote:
Hi,
-Xshare:auto silently ignores failures to map the shared archive
and should be safe to use. I think you're thinking of -Xshare:on
which will fail/abort the VM if mapping fails.
Ah okay.
In that case seems reasonable. But please test thoroughly across
all platforms in JPRT.
Thanks,
David
/Claes
On 2017-10-03 12:28, David Holmes wrote:
Hi Magnus,
As I just put in the bug report, it isn't quite this simple.
You have to be able to tolerate/recover from failure to map the
shared archive.
Cheers,
David
On 3/10/2017 8:24 PM, Magnus Ihse Bursie wrote:
We should use CDS to minimize Java startup time during build.
We run multiple Java commands, and every second counts.
On my machine, I get a ~3% build time speedup with this fix.
Bug: https://bugs.openjdk.java.net/browse/JDK-8188312
WebRev:
http://cr.openjdk.java.net/~ihse/JDK-8188312-use-CDS-for-bootjdk/webrev.01
/Magnus
