quic: refactor QUICHE flag implementation by lkpdn · Pull Request #18485 · envoyproxy/envoy

lkpdn · 2021-10-06T14:47:08Z

QUICHE QUIC_FLAG implementation should be backed by Envoy runtime features not only because it's the canonical way (*1) but also for performance reasons.

It is observed that some quiche::TypedFlag access on the QUICHE datapath becomes a major scaling bottleneck when downstream QUIC traffic is heavy.

Simply replacing absl::MutexLock in TypedFlag implementation with absl::(Reader|Writer)MutexLock where possible is not satisfactory. The bottleneck would just move from the contention on the MutexLock to the atomic operation on the refcount in the new ReaderMutexLock done by each Envoy worker thread. We need a truly scalable flag accessing. Unfortunately, Envoy::Runtime::runtimeFeatureEnabled is also not contention-free since the snapshot created by createNewSnapshot() is shared by every worker thread, which again leads to the refcount contention.

~~With this patch~~ With this PR and #18601, Envoy QUIC performance scales quite well. Please see the attached plot of the scalability comparison between Envoy (as a) QUIC server with-and-without this patch.

(*1) https://github.com/envoyproxy/envoy/blob/1b9c688997f5/source/common/quic/platform/quiche_flags_impl.h#L22-L24

(As a side note, added on 2021/10/11)

Envoy::Runtime::runtimeFeatureEnabled is also not contention-free since the snapshot created by createNewSnapshot() is shared by every worker thread, which again leads to the refcount contention.

This is the reason why do we not only make QUICHE flag implementation backed by Envoy runtime features, but also "unshare" SnapshotImpl. But the contention really occurs? how to reproduce it? The way to reproduce it and its result are shown below:

How to reproduce it?
- build envoy server for two commits
  - 56152d2 and d4f01ac
  - The latter just "unshares" SnapshotImpl, on top of the former commit.
  - The reason I use d4f01ac instead of 87dffe0 is that it's a tiny diff and helps clarify the point. Rest assured that 87dffe0 behaves well at the same level as d4f01ac.
  - Build bazel.debug.server_only with -O2 optimization (my ad-hoc diff: https://gist.github.com/lkpdn/cd416dbcb1a2dba061389bb8eff07f5c)
- environment
  - launch two AWS EC2 c6gn.16xlarge instances (1 client + 1 server)
- prepare a config file
  - see https://gist.github.com/lkpdn/170fac3ab5bbea3409ca79e58123f697
- start an Envoy server with the config above
- pin each worker thread onto a cpu core
- stress the server from the client, using h2load (https://github.com/nghttp2/nghttp2/tree/quic)
```
$ sudo docker run --rm -t --network=host localhost/h2load-quic -c 64 -t 64 -m 800 --warm-up-time=40s -D 40s --tls13-ciphers=TLS_AES_128_GCM_SHA256 --npn-list h3 --connect-to {{ server_ip }}:443 https://127.0.0.1/f1m.dat
```
- run perf-record on the server side while stressing
```
$ sudo perf record -g -t {{ a worker tid }} -- sleep 15
```

Results

flamegraph
- 56152d2
- d4f01ac

zoom into the hot spot, seen in the former (56152d2)

Samples: 60K of event 'cycles', 4000 Hz, Event count (approx.): 30325457223
Envoy::ThreadLocal::InstanceImpl::SlotImpl::get  /home/ubuntu/quic/envoy-dev/linux/arm64/build_envoy_debug/envoy [Percent: local period]
Percent│
       │
       │
       │    Disassembly of section .text:
       │
       │    0000000003e1a040 <Envoy::ThreadLocal::InstanceImpl::SlotImpl::get()>:
       │    _ZN5Envoy11ThreadLocal12InstanceImpl8SlotImpl3getEv():
       │      stp  x29, x30, [sp, #-48]!
       │      str  x21, [sp, #16] 
       │      stp  x20, x19, [sp, #32]
       │      mov  x29, sp
       │      ldr  w20, [x0, #16]
       │      mov  x19, x8
       │      mov  x0, #0x10000                    // #65536
       │      movk x0, #0x3400
       │      nop  
       │      nop  
       │      mrs  x21, tpidr_el0
       │      ldrb w8, [x21, x0]
       │    ↓ cbz  w8, 80
       │34:   mov  x0, #0x10000                    // #65536
       │      movk x0, #0x33e0
       │      nop  
  0.00 │      nop  
       │      add  x8, x21, x0
       │      ldr  x8, [x8, #8] 
       │      add  x8, x8, x20, lsl #4
       │      ldp  x9, x8, [x8]
       │      stp  x9, x8, [x19]
       │    ↓ cbz  x8, 70
  0.00 │      add  x8, x8, #0x8
 28.44 │60:   ldxr x9, [x8]
 70.41 │      add  x9, x9, #0x1
       │      stxr w10, x9, [x8]
  0.05 │    ↑ cbnz w10, 60
  1.10 │70:   ldp  x20, x19, [sp, #32]
       │      ldr  x21, [sp, #16]
       │      ldp  x29, x30, [sp], #48
       │    ← ret  
       │80:   mov  x0, #0x10000                    // #65536
       │      movk x0, #0x3400
       │      nop

Risk Level: Low
Testing:
Docs Changes:
Release Notes:
Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
[Optional Deprecated:]
[Optional API Considerations:]

repokitteh-read-only · 2021-10-06T14:47:11Z

Hi @lkpdn, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

🐱

Caused by: #18485 was opened by lkpdn.

see: more, trace.

lkpdn · 2021-10-07T04:39:29Z

Forgot to run git pre-push. I've just force-pushed to clear up some formatting issues.
Please review, thanks.

repokitteh-read-only · 2021-10-07T06:17:14Z

CC @envoyproxy/dependency-shepherds: Your approval is needed for changes made to (bazel/.*repos.*\.bzl)|(bazel/dependency_imports\.bzl)|(api/bazel/.*\.bzl)|(.*/requirements\.txt)|(.*\.patch).

🐱

Caused by: #18485 was synchronize by lkpdn.

see: more, trace.

alyssawilk · 2021-10-07T12:41:42Z

Awesome find! Mind taking a look at the presubmits and seeing if you can sort them out?
/wait

lkpdn · 2021-10-07T14:11:46Z

I'll look into it. Before that, let me rebase to the latest main branch.

mattklein123 · 2021-10-07T15:08:20Z

quick question: what is the contention that this part of the change solves? It's not immediately clear to me whether the contention is on just referencing the shared_ptr object (seems unlikely) or something internal to the snapshot? Can you add more details?

It's the former that seemed unlikely to me too. Each thread has a local slot in thread_local_data_.data_, though the stored snapshot is shared among them so the contention happens.

But contention on what? Actually accessing the shared_ptr just to return a reference? I would be curious to see a perf trace about this part specifically, because I would prefer to not change this unless it really provides a lot of benefit and if so we should understand it more.

I've just added a side note in this PR description. Could you take a look a it?

Thanks for the perf trace! It looks like InstanceImpl::SlotImpl::getWorker(uint32_t index) has heavy contention. @jmarantz could take a look to see if the contention is expected?

These are my opinion:

One thing I wonder is whether we could have an access variant that does not incref if we know it's safe

If each thread had a weak_ptr to a new snapshot, when the next createNewSnapshot() invoked and the old one destructor runs, each thread may have a dangling pointer to them until it'll be replaced with a new one.

the runtime system appears to be using the untyped variants of TLS slots. It would be great to change them to use the typed versions and eliminate dynamic casts.

I didn't consider this.

A good pattern is to use them when the code being locked is long-running (ie more than just returning a shared_ptr).

I agree.

But if I'm not mistaken this change creates separate shared pointers for every worker all pointing to the same snapshot instance. And this may cause races.

It creates separate shared pointers for every worker all pointing to the "copied" snapshot instance. (So, essentially It does not need to be shared_ptr now, but if we change the part in this PR, the diff would be too large I guess.)

It creates separate shared pointers for every worker all pointing to the "copied" snapshot instance. (So, essentially It does not need to be shared_ptr now, but if we change the part in this PR, the diff would be too large I guess.)

I see, I misread make_shared as shared_ptr. Can this part be split into a separate PR?

I think I'm suggesting something slightly different if this is actually a contention issue:

Still share the data via shared_ptr.

Have a variant which effectively returns the reference *shared_ptr.get().

We know this is safe because of the RCU like semantics as long as the reference is only used during that function call.

I think this should avoid any atomic interaction.

Either way I agree with @rojkov let's please split this part out and discuss separately?

Thank you for the comment. I'm not sure I understand your idea correctly so let me ask a few questions:

Coud you explain "RCU like semantics" in detail?

Looking at the second point you mentioned, I thought the idea is to avoid dynamic_pointer_cast from LoaderImpl::threadSnapshot() and change SnapshotConstSharedPtr to a more plain reference. Am I correct?

But anyway,

Either way I agree with @rojkov let's please split this part out and discuss separately?

Alright, I'll split this part out, leaving the "QUIC flags backed by runtime features" part in this PR 18485.

@mattklein123 @rojkov @jmarantz @danzh2010
Please see #18601 .

danzh2010 · 2021-10-07T15:34:35Z

We sometimes use this in tests, can we just implement it somehow?

56152d2 removed those call sites. And I think it should be done with Runtime::LoaderSingleton::getExisting()->mergeValues when some flags need to be tweaked in tests.

Sorry I was wrong. I didn't remove SetQuicheReloadableFlagImpl call sites in tests at all.
To be honest, I don't see any merit keeping these set/get testing if the quic flag implementation is to be integrated to Envoy runtime feature infrastructure.

We do need SetQuicheReloadableFlagImpl() sometimes even in non-test code in case where Envoy needs a feature/fix in QUICHE which is not turned on yet.

Okay I got it. I'll implement it. Thank you.

lkpdn · 2021-10-08T07:09:17Z

I looked into the presubmits failures, then I noticed that the copy constructor implementation of SnapshotImpl that I added was incomplete. Almost all failures looked like due to the empty EntryMap value_ (but of course generator_ and stats_ were incorrectly missing).

Rather than introducing the copy construction and messing the code base, just calling createNewSnapshot for each thread looks simple, so I pushed d4f01ac.

lkpdn · 2021-10-08T10:15:13Z

It seems that multiple createNewSnapshot calls cause RuntimeStats' load_(success|error) counters to be incorrectly incremented multiple times, leading to the new presubmit failures. To summarize the current situtation,

The upstream implementation shares the whole SnapshotImpl, which is one of the causes of the scalability issue.
In this PR, the third commit tries to unshare the SnapshotImpl.
- (1st try) 87dffe0 tried to copy the whole structure for each thread. Due to my incomplete implementation of SnapshotImpl copy constructor, it caused some failures.
- (2nd try) d4f01ac also intends to copy the whole structure. Again, I noticed after I git-pushed it that it's problematic around RuntimeStats' load_(success|error) counting.

After thinking carefully, I realized that only EntryMap values_ needs to be unshared from performance perspective. I will make a patch (3rd try, in a more careful manner). I'd appreciate if you could give me some help/advice though.

danzh2010 · 2021-10-11T15:56:21Z

/assign @RenjieTang who implemented QUICHE flags with Envoy runtime.

RenjieTang · 2021-10-11T16:55:56Z

Wow an interesting find! Thanks for fixing this.

jmarantz · 2021-10-11T18:20:39Z

Before diving in I want to say this is one of the crispest, most detailed, data-driven PR descriptions I have ever seen. Nice.

The currently used QUICHE for envoy integration already contains the upstream commit 68b06f23af21 ("Refactor QUICHE platform flag accessors.") so these accessor implementations are no longer needed. Signed-off-by: Koichiro Den <den@valinux.co.jp>

alyssawilk · 2021-10-12T13:39:45Z

side note, please don't force push as it complicates the review process. Please merge from main rather than rebase for updates. thanks!

- As the flags are now backed by Envoy Runtime Configuration, updating those values online from inside should be prohibited. They should just be updated in the same way as other runtime features. - QUIC_PROTOCOL_FLAG implementation is converted to plain global variables. Signed-off-by: Koichiro Den <den@valinux.co.jp>

Signed-off-by: Koichiro Den <den@valinux.co.jp>

…-access-contention Signed-off-by: Koichiro Den <den@valinux.co.jp>

lkpdn · 2021-10-13T15:32:22Z

@mattklein123 Alright, done.

lkpdn · 2021-10-13T15:39:45Z

Note that the reason I added a commit 850bb91 is not because I added some new code that is not covered by testing. It just seems that deleting some covered lines makes the final result down to 95.9%.

danzh2010

Thanks for working on this! I left a few nits, but looks good overall!
@RenjieTang, do you mind take a look as well?

danzh2010 · 2021-10-13T16:51:21Z

        "//envoy/runtime:runtime_interface",
        "//source/common/common:hash_lib",
        "//source/common/singleton:const_singleton",
+        "@com_github_google_quiche//:quic_core_flags_list_lib",


You will need envoy_select_enable_http3() for this.

Indeed. I'll push a fix shortly. Thank you!

Just noticed that the quic platform tests had been executed successfully even when http3=False build.
quiche::FlagRegistry was holding quic flags even if http3=False.
So, in order to make it run even after making it backed by runtime features, in 3a13a02 I reverted this part.

danzh2010 · 2021-10-13T16:57:18Z

  testNewOverrides(*loader_, store_);
 }

-#ifdef ENVOY_ENABLE_QUIC


Why is this test removed?

I just forgot to revert this dropping after #18485 (comment). Just my mistake. I'll push a fix, thanks!

In e4d8613, I modified and moved this test under RtdsLoaderImplTest . Now I wonder this PR somewhat would break backward compatibility. Some admin might expect they can override the QUICHE reloadable/restart flags from any layer.

Is this why you added admin layer and called doOnConfigUpdateVerifyNoThrow(runtime, 0) in the test? I'm not familiar with the control plan, but updating flags from admin layer only seems fine to me. @htuch

I think if something is a runtime flag, it should ideally be updatable from any runtime layer, otherwise we violate principle-of-least-surprise (every other runtime flag should have this capability). Is there a reason we can't do this for QUIC?

My comment #18485 (comment) was somewhat perplexing.

It's updatable from any runtime layer. The point is that QUICHE flag could be updated at an arbitrary timing from inside, on admin layer, if such an implementation will be merged. That's why I dropped the SetQuiche(Reloadable|Restart)FlagsImpl capability in my original PR (ref. #18485 (comment)) though.

The current upstream implementation calls SetQuic(Reloadable|Restart)Flag from inside (e.g., on ActiveQuicListener instantiation). With this PR, those places will be eliminated, except tests.

So, basically administrators can update those flags from any runtime layer without worry.

In that sense, in the SetQuiche(Reloadable|Restart)FlagImpl implementation, replacing ASSERT(Envoy::Thread::MainThread::isMainOrTestThread()); with something like ASSERT(Envoy::Thread::MainThread::isTestThread()); might help.

SG - as long as we are treating the flags uniformly with other runtime flags then no objections.

Ref: #18485 (review) Signed-off-by: Koichiro Den <den@valinux.co.jp>

lkpdn · 2021-10-14T11:40:23Z

@danzh2010 Thank you for the review!

Just pushed e4d8613.
PTAL, thanks.

Signed-off-by: Koichiro Den <den@valinux.co.jp>

…-access-contention Signed-off-by: Koichiro Den <den@valinux.co.jp>

…if http3=False Signed-off-by: Koichiro Den <den@valinux.co.jp>

lkpdn · 2021-10-15T08:36:57Z

The coverage failure looks similar to

danzh2010

LGTM

danzh2010 · 2021-10-15T16:57:00Z

 # for existing directories with low coverage.
 declare -a KNOWN_LOW_COVERAGE=(
-"source/common:95.9" # Raise when QUIC coverage goes up
+"source/common:96.0" # Raise when QUIC coverage goes up


nice! Does source/common/quic/ coverage changed?

The latest CI coverage run failed in bazel coverage stage so the actual output (artifact, including logs) is not available.

But anyway, #18591 (ref. #17479 (comment)) was merged a few days ago, so I think we better to re-run and look at the result, with the new 95.9% threshold being untouched.

danzh2010 · 2021-10-15T17:29:13Z

  testNewOverrides(*loader_, store_);
 }

-#ifdef ENVOY_ENABLE_QUIC


Is this why you added admin layer and called doOnConfigUpdateVerifyNoThrow(runtime, 0) in the test? I'm not familiar with the control plan, but updating flags from admin layer only seems fine to me. @htuch

Signed-off-by: Koichiro Den <den@valinux.co.jp>

lkpdn · 2021-10-18T14:50:30Z

We got two new failures even though a9d5b2b is tiny diff.

linux_x64 release https://dev.azure.com/cncf/envoy/_build/results?buildId=92194&view=logs&j=8c169225-0ae8-53bd-947f-07cb81846cb5&t=6b0ace90-28b2-51d9-2951-b1fd35e67dec&l=88 : looks irrelevant to this PR. I cannot reproduce this locally.
linux_arm64 release https://dev.azure.com/cncf/envoy/_build/results?buildId=92194&view=logs&j=767be981-567e-57d8-68c3-2140ede0a0bd&t=1fb0b72f-6293-562d-a4d1-bb649223eaca&l=18128 : I cannot reproduce this locally + the segfault backtrace seen in the link gives me no clue. Looks relevant (because it's about quic) so it might be that this becomes flaky because of this PR. Will dig into this one.

Signed-off-by: Koichiro Den <den@valinux.co.jp>

lkpdn · 2021-10-19T10:40:13Z

https://dev.azure.com/cncf/envoy/_build/results?buildId=92264&view=logs&j=bbe4b42d-86e6-5e9c-8a0b-fea01d818a24&t=e00c5a13-c6dc-5e9a-6104-69976170e881&l=11105
I'm not sure this is relevant to this PR diff. 🤔

danzh2010 · 2021-10-20T22:50:40Z

                            Network::Address::InstanceConstSharedPtr local_addr,
                            QuicStatNames& quic_stat_names, Stats::Scope& scope) {
-  // This flag fix a QUICHE issue which may crash Envoy during connection close.
-  SetQuicReloadableFlag(quic_single_ack_in_packet2, true);


Does Envoy runtime work for client side? @ggreenway
If not, we still need to set this flag here and quiche::FlagRegistry::getInstance() above.

Answer to myself, yes, RuntimeFeature is a singleton instantiated upon fetching a feature.

github-actions · 2021-11-25T20:01:23Z

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in 7 days if no further activity occurs. Please feel free to give a status update now, ping for review, or re-open when it's ready. Thank you for your contributions!

github-actions · 2021-12-02T20:01:41Z

This pull request has been automatically closed because it has not had activity in the last 37 days. Please feel free to give a status update now, ping for review, or re-open when it's ready. Thank you for your contributions!

mattklein123 assigned alyssawilk and danzh2010 Oct 6, 2021

lkpdn force-pushed the eliminate-quic-flag-access-contention branch from c47c097 to 375fe66 Compare October 7, 2021 04:36

lkpdn force-pushed the eliminate-quic-flag-access-contention branch from 375fe66 to 8b9408a Compare October 7, 2021 06:17

repokitteh-read-only Bot added the deps Approval required for changes to Envoy's external dependencies label Oct 7, 2021

lkpdn force-pushed the eliminate-quic-flag-access-contention branch from 8b9408a to 58b0355 Compare October 7, 2021 09:56

repokitteh-read-only Bot added the waiting label Oct 7, 2021

lkpdn force-pushed the eliminate-quic-flag-access-contention branch from 58b0355 to 87dffe0 Compare October 7, 2021 14:13

repokitteh-read-only Bot removed the waiting label Oct 7, 2021

mattklein123 reviewed Oct 7, 2021

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danzh2010 reviewed Oct 7, 2021

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lkpdn force-pushed the eliminate-quic-flag-access-contention branch from 87dffe0 to d4f01ac Compare October 8, 2021 06:44

lkpdn force-pushed the eliminate-quic-flag-access-contention branch 2 times, most recently from 192a16f to be7ec15 Compare October 11, 2021 13:45

repokitteh-read-only Bot assigned RenjieTang Oct 11, 2021

lkpdn force-pushed the eliminate-quic-flag-access-contention branch from be7ec15 to d0186f0 Compare October 12, 2021 02:37

lkpdn force-pushed the eliminate-quic-flag-access-contention branch 2 times, most recently from 0ccb9d9 to bdbfd1c Compare October 12, 2021 10:08

Koichiro Den added 3 commits October 14, 2021 00:30

quic: adjusting coverage (#18485)

850bb91

Signed-off-by: Koichiro Den <den@valinux.co.jp>

Merge remote-tracking branch 'upstream/main' into eliminate-quic-flag…

3786431

…-access-contention Signed-off-by: Koichiro Den <den@valinux.co.jp>

lkpdn force-pushed the eliminate-quic-flag-access-contention branch from e22e06e to 3786431 Compare October 13, 2021 15:31

repokitteh-read-only Bot removed the waiting label Oct 13, 2021

danzh2010 reviewed Oct 13, 2021

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quic: fixes/changes after review by danzh

e4d8613

Ref: #18485 (review) Signed-off-by: Koichiro Den <den@valinux.co.jp>

Koichiro Den added 2 commits October 14, 2021 13:48

quic: make resetQuicheProtocolFlags run as a constructor

2c4af15

Signed-off-by: Koichiro Den <den@valinux.co.jp>

Merge remote-tracking branch 'upstream/main' into eliminate-quic-flag…

7fbb32d

…-access-contention Signed-off-by: Koichiro Den <den@valinux.co.jp>

mattklein123 removed their assignment Oct 14, 2021

quic: resolve win issue + make platform test runnable as before even …

3a13a02

…if http3=False Signed-off-by: Koichiro Den <den@valinux.co.jp>

danzh2010 previously approved these changes Oct 15, 2021

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lkpdn commented Oct 16, 2021

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Comment thread test/common/runtime/runtime_impl_test.cc Outdated

quic: fixes/changes after 2nd review by danzh

a9d5b2b

Signed-off-by: Koichiro Den <den@valinux.co.jp>

lkpdn dismissed danzh2010’s stale review via a9d5b2b October 18, 2021 09:54

Koichiro Den and others added 2 commits October 19, 2021 01:38

quic: add safety guard to the new QUICHE flag setter implementation

e5018b5

Signed-off-by: Koichiro Den <den@valinux.co.jp>

Merge branch 'main' into eliminate-quic-flag-access-contention

84e0c76

Merge branch 'main' into eliminate-quic-flag-access-contention

b605e8b

danzh2010 reviewed Oct 20, 2021

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alyssawilk added the waiting label Oct 26, 2021

github-actions Bot added the stale stalebot believes this issue/PR has not been touched recently label Nov 25, 2021

github-actions Bot closed this Dec 2, 2021

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