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  1. 29 Jul, 2015 8 commits
    • Russ Cox's avatar
      cmd/compile: align arm64 stack frames correctly · 3952057c
      Russ Cox authored 
      If the compiler doesn't do it, cmd/internal/obj/arm64 will,
and that will break the zeroing of ambiguously live values
done in zerorange, which in turn produces uninitialized
pointer cells that the GC trips over.

For #9880.

Change-Id: Ice97c30bc8b36d06b7b88d778d87fab8e1827fdc
Reviewed-on: https://go-review.googlesource.com/12847Reviewed-by: default avatarAustin Clements <austin@google.com>
      3952057c
    • Austin Clements's avatar
      runtime: report GC CPU utilization in MemStats · 23e4744c
      Austin Clements authored 
      This adds a GCCPUFraction field to MemStats that reports the
cumulative fraction of the program's execution time spent in the
garbage collector. This is equivalent to the utilization percent shown
in the gctrace output and makes this available programmatically.

This does make one small effect on the gctrace output: we now report
the duration of mark termination up to just before the final
start-the-world, rather than up to just after. However, unlike
stop-the-world, I don't believe there's any way that start-the-world
can block, so it should take negligible time.

While there are many statistics one might want to expose via MemStats,
this is one of the few that will undoubtedly remain meaningful
regardless of future changes to the memory system.

The diff for this change is larger than the actual change. Mostly it
lifts the code for computing the GC CPU utilization out of the
debug.gctrace path.

Updates #10323.

Change-Id: I0f7dc3fdcafe95e8d1233ceb79de606b48acd989
Reviewed-on: https://go-review.googlesource.com/12844Reviewed-by: default avatarRuss Cox <rsc@golang.org>
      23e4744c
    • Austin Clements's avatar
      runtime: always capture GC phase transition times · 4b71660c
      Austin Clements authored 
      Currently we only capture GC phase transition times if
debug.gctrace>0, but we're about to compute GC CPU utilization
regardless of whether debug.gctrace is set, so we need these
regardless of debug.gctrace.

Change-Id: If3acf16505a43d416e9a99753206f03287180660
Reviewed-on: https://go-review.googlesource.com/12843Reviewed-by: default avatarRuss Cox <rsc@golang.org>
Reviewed-by: default avatarRick Hudson <rlh@golang.org>
      4b71660c
    • Austin Clements's avatar
      runtime: avoid race between SIGPROF traceback and stack barriers · 87f97c73
      Austin Clements authored 
      The following sequence of events can lead to the runtime attempting an
out-of-bounds access on a stack barrier slice:

1. A SIGPROF comes in on a thread while the G on that thread is in
   _Gsyscall. The sigprof handler calls gentraceback, which saves a
   local copy of the G's stkbar slice. Currently the G has no stack
   barriers, so this slice is empty.

2. On another thread, the GC concurrently scans the stack of the
   goroutine being profiled (it considers it stopped because it's in
   _Gsyscall) and installs stack barriers.

3. Back on the sigprof thread, gentraceback comes across a stack
   barrier in the stack and attempts to look it up in its (zero
   length) copy of G's old stkbar slice, which causes an out-of-bounds
   access.

This commit fixes this by adding a simple cas spin to synchronize the
SIGPROF handler with stack barrier insertion.

In general I would prefer that this synchronization be done through
the G status, since that's how stack scans are otherwise synchronized,
but adding a new lock is a much smaller change and G statuses are full
of subtlety.

Fixes #11863.

Change-Id: Ie89614a6238bb9c6a5b1190499b0b48ec759eaf7
Reviewed-on: https://go-review.googlesource.com/12748Reviewed-by: default avatarRuss Cox <rsc@golang.org>
      87f97c73
    • Rick Hudson's avatar
      runtime: force mutator to give work buffer to GC · e95bc5fe
      Rick Hudson authored 
      The scheduler, work buffer's dispose, and write barriers
can conspire to hide the a pointer from the GC's concurent
mark phase. If this pointer is the only path to a large
amount of marking the STW mark termination phase may take
a lot of time.

Consider the following:
1) dispose places a work buffer on the partial queue
2) the GC is busy so it does not immediately remove and
   process the work buffer
3) the scheduler runs a mutator whose write barrier dequeues the
   work buffer from the partial queue so the GC won't see it
This repeats until the GC reaches the mark termination
phase where the GC finally discovers the pointer along
with a lot of work to do.

This CL fixes the problem by having the mutator
dispose of the buffer to the full queue instead of
the partial queue. Since the write buffer never asks for full
buffers the conspiracy described above is not possible.

Updates #11694.

Change-Id: I2ce832f9657a7570f800e8ce4459cd9e304ef43b
Reviewed-on: https://go-review.googlesource.com/12840Reviewed-by: default avatarAustin Clements <austin@google.com>
      e95bc5fe
    • Rob Pike's avatar
      doc: add json tokenizer to go1.5.html · ff991940
      Rob Pike authored 
      Change-Id: I45d92fed757fa1866d5b80e53ed1af6712fa6741
Reviewed-on: https://go-review.googlesource.com/12782Reviewed-by: default avatarRuss Cox <rsc@golang.org>
      ff991940
    • Mikio Hara's avatar
      cmd/internal/asm: delete · 861af804
      Mikio Hara authored 
      Updates #10510.

Change-Id: Ib4d39943969d18517b373292b83d87650d5df12a
Reviewed-on: https://go-review.googlesource.com/12787
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: default avatarBrad Fitzpatrick <bradfitz@golang.org>
      861af804
    • Rob Pike's avatar
      cmd: delete old[5689]a · ba0c142d
      Rob Pike authored 
      These are the old assemblers written in C, and now they are
not needed.

Fixes #10510.

Change-Id: Id9337ffc8eccfd93c84b2e23f427fb1a576b543d
Reviewed-on: https://go-review.googlesource.com/12784Reviewed-by: default avatarRuss Cox <rsc@golang.org>
Reviewed-by: default avatarBrad Fitzpatrick <bradfitz@golang.org>
      ba0c142d
  3. 28 Jul, 2015 23 commits
  5. 27 Jul, 2015 9 commits
    • Brad Fitzpatrick's avatar
      net/http: pause briefly after closing Server connection when body remains · a3ffd836
      Brad Fitzpatrick authored 
      From https://github.com/golang/go/issues/11745#issuecomment-123555313
this implements option (b), having the server pause slightly after
sending the final response on a TCP connection when we're about to close
it when we know there's a request body outstanding. This biases the
client (which might not be Go) to prefer our response header over the
request body write error.

Updates #11745

Change-Id: I07cb0b74519d266c8049d9e0eb23a61304eedbf8
Reviewed-on: https://go-review.googlesource.com/12658Reviewed-by: default avatarRuss Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
      a3ffd836
    • David Crawshaw's avatar
      runtime/cgo: remove TMPDIR logic for iOS · 249894ab
      David Crawshaw authored 
      Seems like the simplest solution for 1.5. All the parts of the test
suite I can run on my current device (for which my exception handler
fix no longer works, apparently) pass without this code. I'll move it
into x/mobile/app.

Fixes #11884

Change-Id: I2da40c8c7b48a4c6970c4d709dd7c148a22e8727
Reviewed-on: https://go-review.googlesource.com/12721Reviewed-by: default avatarIan Lance Taylor <iant@golang.org>
      249894ab
    • Austin Clements's avatar
      runtime: close window that hides GC work from concurrent mark · c1f7a56f
      Austin Clements authored 
      Currently we enter mark 2 by first flushing all existing gcWork caches
and then setting gcBlackenPromptly, which disables further gcWork
caching. However, if a worker or assist pulls a work buffer in to its
gcWork cache after that cache has been flushed but before caching is
disabled, that work may remain in that cache until mark termination.
If that work represents a heap bottleneck (e.g., a single pointer that
is the only way to reach a large amount of the heap), this can force
mark termination to do a large amount of work, resulting in a long
STW.

Fix this by reversing the order of these steps: first disable caching,
then flush all existing caches.

Rick Hudson <rlh> did the hard work of tracking this down. This CL
combined with CL 12672 and CL 12646 distills the critical parts of his
fix from CL 12539.

Fixes #11694.

Change-Id: Ib10d0a21e3f6170a80727d0286f9990df049fed2
Reviewed-on: https://go-review.googlesource.com/12688Reviewed-by: default avatarRick Hudson <rlh@golang.org>
      c1f7a56f
    • Austin Clements's avatar
      runtime: enable GC assists ASAP · 510fd135
      Austin Clements authored 
      Currently the GC coordinator enables GC assists at the same time it
enables background mark workers, after the concurrent scan phase is
done. However, this means a rapidly allocating mutator has the entire
scan phase during which to allocate beyond the heap trigger and
potentially beyond the heap goal with no back-pressure from assists.
This prevents the feedback system that's supposed to keep the heap
size under the heap goal from doing its job.

Fix this by enabling mutator assists during the scan phase. This is
safe because the write barrier is already enabled and globally
acknowledged at this point.

There's still a very small window between when the heap size reaches
the heap trigger and when the GC coordinator is able to stop the world
during which the mutator can allocate unabated. This allows *very*
rapidly allocator mutators like TestTraceStress to still occasionally
exceed the heap goal by a small amount (~20 MB at most for
TestTraceStress). However, this seems like a corner case.

Fixes #11677.

Change-Id: I0f80d949ec82341cd31ca1604a626efb7295a819
Reviewed-on: https://go-review.googlesource.com/12674Reviewed-by: default avatarRuss Cox <rsc@golang.org>
      510fd135
    • Austin Clements's avatar
      runtime: allow GC drain whenever write barrier is enabled · f5e67e53
      Austin Clements authored 
      Currently we hand-code a set of phases when draining is allowed.
However, this set of phases is conservative. The critical invariant is
simply that the write barrier must be enabled if we're draining.

Shortly we're going to enable mutator assists during the scan phase,
which means we may drain during the scan phase. In preparation, this
commit generalizes these assertions to check the fundamental condition
that the write barrier is enabled, rather than checking that we're in
any particular phase.

Change-Id: I0e1bec1ca823d4a697a0831ec4c50f5dd3f2a893
Reviewed-on: https://go-review.googlesource.com/12673Reviewed-by: default avatarRuss Cox <rsc@golang.org>
      f5e67e53
    • Austin Clements's avatar
      runtime: don't start workers between mark 1 & 2 · 64a32ffe
      Austin Clements authored 
      Currently we clear both the mark 1 and mark 2 signals at the beginning
of concurrent mark. If either if these is clear, it acts as a signal
to the scheduler that it should start background workers. However,
this means that in the interim *between* mark 1 and mark 2, the
scheduler basically loops starting up new workers only to have them
return with nothing to do. In addition to harming performance and
delaying mutator work, this approach has a race where workers started
for mark 1 can mistakenly signal mark 2, causing it to complete
prematurely. This approach also interferes with starting assists
earlier to fix #11677.

Fix this by initially setting both mark 1 and mark 2 to "signaled".
The scheduler will not start background mark workers, though assists
can still run. When we're ready to enter mark 1, we clear the mark 1
signal and wait for it. Then, when we're ready to enter mark 2, we
clear the mark 2 signal and wait for it.

This structure also lets us deal cleanly with the situation where all
work is drained *prior* to the mark 2 wait, meaning that there may be
no workers to signal completion. Currently we deal with this using a
racy (and possibly incorrect) check for work in the coordinator itself
to skip the mark 2 wait if there's no work. This change makes the
coordinator unconditionally wait for mark completion and makes the
scheduler itself signal completion by slightly extending the logic it
already has to determine that there's no work and hence no use in
starting a new worker.

This is a prerequisite to fixing the remaining component of #11677,
which will require enabling assists during the scan phase. However, we
don't want to enable background workers until the mark phase because
they will compete with the scan. This change lets us use bgMark1 and
bgMark2 to indicate when it's okay to start background workers
independent of assists.

This is also a prerequisite to fixing #11694. It significantly reduces
the occurrence of long mark termination pauses in #11694 (from 64 out
of 1000 to 2 out of 1000 in one experiment).

Coincidentally, this also reduces the final heap size (and hence run
time) of TestTraceStress from ~100 MB and ~1.9 seconds to ~14 MB and
~0.4 seconds because it significantly shortens concurrent mark
duration.

Rick Hudson <rlh> did the hard work of tracking this down.

Change-Id: I12ea9ee2db9a0ae9d3a90dde4944a75fcf408f4c
Reviewed-on: https://go-review.googlesource.com/12672Reviewed-by: default avatarRuss Cox <rsc@golang.org>
      64a32ffe
    • Austin Clements's avatar
      runtime: retry GC assist until debt is paid off · 8f34b253
      Austin Clements authored 
      Currently, there are three ways to satisfy a GC assist: 1) the mutator
steals credit from background GC, 2) the mutator actually does GC
work, and 3) there is no more work available. 3 was never really
intended as a way to satisfy an assist, and it causes problems: there
are periods when it's expected that the GC won't have any work, such
as when transitioning from mark 1 to mark 2 and from mark 2 to mark
termination. During these periods, there's no back-pressure on rapidly
allocating mutators, which lets them race ahead of the heap goal.

For example, test/init1.go and the runtime/trace test both have small
reachable heaps and contain loops that rapidly allocate large garbage
byte slices. This bug lets these tests exceed the heap goal by several
orders of magnitude.

Fix this by forcing the assist (and hence the allocation) to block
until it can satisfy its debt via either 1 or 2, or the GC cycle
terminates.

This fixes one the causes of #11677. It's still possible to overshoot
the GC heap goal, but with this change the overshoot is almost exactly
by the amount of allocation that happens during the concurrent scan
phase, between when the heap passes the GC trigger and when the GC
enables assists.

Change-Id: I5ef4edcb0d2e13a1e432e66e8245f2bd9f8995be
Reviewed-on: https://go-review.googlesource.com/12671Reviewed-by: default avatarRuss Cox <rsc@golang.org>
      8f34b253
    • Austin Clements's avatar
      runtime: yield to GC coordinator after assist completion · 500c88d4
      Austin Clements authored 
      Currently it's possible for the GC assist to signal completion of the
mark phase, which puts the GC coordinator goroutine on the current P's
run queue, and then return to mutator code that delays until the next
forced preemption before actually yielding control to the GC
coordinator, dragging out completion of the mark phase. This delay can
be further exacerbated if the mutator makes other goroutines runnable
before yielding control, since this will push the GC coordinator on
the back of the P's run queue.

To fix this, this adds a Gosched to the assist if it completed the
mark phase. This immediately and directly yields control to the GC
coordinator. This already happens implicitly in the background mark
workers because they park immediately after completing the mark.

This is one of the reasons completion of the mark phase is being
dragged out and allowing the mutator to allocate without assisting,
leading to the large heap goal overshoot in issue #11677. This is also
a prerequisite to making the assist block when it can't pay off its
debt.

Change-Id: I586adfbecb3ca042a37966752c1dc757f5c7fc78
Reviewed-on: https://go-review.googlesource.com/12670Reviewed-by: default avatarRuss Cox <rsc@golang.org>
      500c88d4
    • Austin Clements's avatar
      runtime: disallow GC assists in non-preemptible contexts · 4f188c2d
      Austin Clements authored 
      Currently it's possible to perform GC work on a system stack or when
locks are held if there's an allocation that triggers an assist. This
is generally a bad idea because of the fragility of these contexts,
and it's incompatible with two changes we're about to make: one is to
yield after signaling mark completion (which we can't do from a
non-preemptible context) and the other is to make assists block if
there's no other way for them to pay off the assist debt.

This commit simply skips the assist if it's called from a
non-preemptible context. The allocation will still count toward the
assist debt, so it will be paid off by a later assist. There should be
little allocation from non-preemptible contexts, so this shouldn't
harm the overall assist mechanism.

Change-Id: I7bf0e6c73e659fe6b52f27437abf39d76b245c79
Reviewed-on: https://go-review.googlesource.com/12649Reviewed-by: default avatarRuss Cox <rsc@golang.org>
      4f188c2d
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