feat(server): support epoll linux api (#351)

.github/ISSUE_TEMPLATE/bug_report.md

@@ -25,7 +25,7 @@ If applicable, add screenshots to help explain your problem.
 
 **Environment (please complete the following information):**
  - OS: [ubuntu 20.04]
- - Kernel: [MUST BE 5.10 or greater] # Command: `uname -a`
+ - Kernel: # Command: `uname -a`
  - Containerized?: [Bare Metal, Docker, Docker Compose, Docker Swarm, Kubernetes, Other]
  - Dragonfly Version: [e.g. 0.3.0]
 

README.md

@@ -81,9 +81,8 @@ For more info about memory efficiency in Dragonfly see [dashtable doc](/docs/das
 
 ## Running the server
 
-Dragonfly runs on linux. It uses relatively new linux specific [io-uring API](https://github.com/axboe/liburing)
+Dragonfly runs on linux. We advice running it on linux version 5.11 or later
-for I/O, hence it requires Linux version 5.10 or later.
+but you can also run Dragonfly on older kernels as well.
-Debian/Bullseye, Ubuntu 20.04.4 or later fit these requirements.
 
 
 ### With docker:
@@ -141,9 +140,9 @@ In addition, it has Dragonfly specific arguments options:
    `keys` is a dangerous command. We truncate its result to avoid blowup in memory when fetching too many keys.
  * `dbnum` - maximum number of supported databases for `select`.
  * `cache_mode` - see [Cache](#novel-cache-design) section below.
- * `hz` - key expiry evaluation frequency. Default is 1000. Lower frequency uses less cpu when 
+ * `hz` - key expiry evaluation frequency. Default is 1000. Lower frequency uses less cpu when
    idle at the expense of precision in key eviction.
- * `save_schedule` - glob spec for the UTC time to save a snapshot which matches HH:MM (24h time). default: "" 
+ * `save_schedule` - glob spec for the UTC time to save a snapshot which matches HH:MM (24h time). default: ""
  * `keys_output_limit` -  Maximum number of keys output by keys command. default: 8192
 
 

docs/build-from-source.md

@@ -2,9 +2,8 @@
 
 ## Running the server
 
-Dragonfly runs on linux. It uses relatively new linux specific [io-uring API](https://github.com/axboe/liburing)
+Dragonfly runs on linux. We advice running it on linux version 5.11 or later
-for I/O, hence it requires `Linux verion 5.10` or later.
+but you can also run Dragonfly on older kernels as well.
-Debian/Bullseye, `Ubuntu 20.04.4` or later fit these requirements.
 
 ### WARNING: Building from source on older kernels WILL NOT WORK.
 
@@ -58,7 +57,7 @@ OK
 1) "hello"
 127.0.0.1:6379> get hello
 "world"
-127.0.0.1:6379> 
+127.0.0.1:6379>
 ```
 
 ## Step 6

docs/quick-start/README.md

@@ -37,7 +37,7 @@ OK
 1) "hello"
 127.0.0.1:6379> get hello
 "world"
-127.0.0.1:6379> 
+127.0.0.1:6379>
 ```
 
 ## Step 3
@@ -46,10 +46,6 @@ Continue being great and build your app with the power of DragonflyDB!
 
 ## Known issues
 
-#### `Error initializing io_uring`
-
-This likely means your kernel version is too low to run DragonflyDB. Make sure to install
-a kernel version that supports `io_uring`.
 
 ## More Build Options
 - [Docker Compose Deployment](/contrib/docker/)

helio

@@ -1 +1 @@
-Subproject commit 723774a0248102f97c8b0e71b5078025386c3ac1
+Subproject commit 65325585b54c748fc5039b18961d5f0972e8e045

src/facade/dragonfly_connection.cc

@@ -22,8 +22,6 @@
 #include "util/tls/tls_socket.h"
 #endif
 
-#include "util/uring/uring_socket.h"
-
 ABSL_FLAG(bool, tcp_nodelay, false,
           "Configures dragonfly connections with socket option TCP_NODELAY");
 ABSL_FLAG(bool, http_admin_console, true, "If true allows accessing http console on main TCP port");
@@ -162,8 +160,8 @@ void Connection::OnShutdown() {
 void Connection::OnPreMigrateThread() {
   // If we migrating to another io_uring we should cancel any pending requests we have.
   if (break_poll_id_ != kuint32max) {
-    uring::UringSocket* us = static_cast<uring::UringSocket*>(socket_.get());
+    auto* ls = static_cast<LinuxSocketBase*>(socket_.get());
-    us->CancelPoll(break_poll_id_);
+    ls->CancelPoll(break_poll_id_);
     break_poll_id_ = kuint32max;
   }
 }
@@ -173,9 +171,9 @@ void Connection::OnPostMigrateThread() {
   if (breaker_cb_) {
     DCHECK_EQ(kuint32max, break_poll_id_);
 
-    uring::UringSocket* us = static_cast<uring::UringSocket*>(socket_.get());
+    auto* ls = static_cast<LinuxSocketBase*>(socket_.get());
     break_poll_id_ =
-        us->PollEvent(POLLERR | POLLHUP, [this](int32_t mask) { this->OnBreakCb(mask); });
+        ls->PollEvent(POLLERR | POLLHUP, [this](int32_t mask) { this->OnBreakCb(mask); });
   }
 }
 
@@ -242,8 +240,7 @@ void Connection::HandleRequests() {
     } else {
       cc_.reset(service_->CreateContext(peer, this));
 
-      // TODO: to move this interface to LinuxSocketBase so we won't need to cast.
+      auto* us = static_cast<LinuxSocketBase*>(socket_.get());
-      uring::UringSocket* us = static_cast<uring::UringSocket*>(socket_.get());
       if (breaker_cb_) {
         break_poll_id_ =
             us->PollEvent(POLLERR | POLLHUP, [this](int32_t mask) { this->OnBreakCb(mask); });

src/server/CMakeLists.txt

@@ -1,5 +1,5 @@
 add_executable(dragonfly dfly_main.cc)
-cxx_link(dragonfly base dragonfly_lib)
+cxx_link(dragonfly base dragonfly_lib epoll_fiber_lib)
 
 if (CMAKE_SYSTEM_PROCESSOR STREQUAL "x86_64" AND CMAKE_BUILD_TYPE STREQUAL "Release")
   # Add core2 only to this file, thus avoiding instructions in this object file that

src/server/dfly_main.cc

@@ -26,6 +26,7 @@
 #include "server/version.h"
 #include "strings/human_readable.h"
 #include "util/accept_server.h"
+#include "util/epoll/epoll_pool.h"
 #include "util/uring/uring_pool.h"
 #include "util/varz.h"
 
@@ -52,6 +53,10 @@ ABSL_FLAG(string, unixsocket, "",
           "If not empty - specifies path for the Unis socket that will "
           "be used for listening for incoming connections.");
 
+ABSL_FLAG(bool, force_epoll, false,
+          "If true - uses linux epoll engine underneath."
+          "Can fit for kernels older than 5.10.");
+
 using namespace util;
 using namespace facade;
 using namespace io;
@@ -193,6 +198,42 @@ bool CreatePidFile(const string& path) {
   return true;
 }
 
+bool ShouldUseEpollAPI(const base::sys::KernelVersion& kver) {
+  if (GetFlag(FLAGS_force_epoll))
+    return true;
+
+  if (kver.kernel < 5 || (kver.kernel == 5 && kver.major < 10)) {
+    LOG(WARNING) << "Kernel is older than 5.10, switching to epoll engine.";
+    return true;
+  }
+
+  struct io_uring ring;
+  io_uring_params params;
+  memset(&params, 0, sizeof(params));
+
+  int iouring_res = io_uring_queue_init_params(1024, &ring, &params);
+
+  if (iouring_res == 0) {
+    io_uring_queue_exit(&ring);
+    return false;
+  }
+
+  iouring_res = -iouring_res;
+
+  if (iouring_res == ENOSYS) {
+    LOG(WARNING) << "iouring API is not supported. switching to epoll.";
+  } else if (iouring_res == ENOMEM) {
+    LOG(WARNING) << "io_uring does not have enough memory. That can happen when your "
+                    "max locked memory is too limited. If you run via docker, "
+                    "try adding '--ulimit memlock=-1' to \"docker run\" command."
+                    "Meanwhile, switching to epoll";
+  } else {
+    LOG(WARNING) << "Weird error " << iouring_res << " switching to epoll";
+  }
+
+  return true;
+}
+
 }  // namespace
 }  // namespace dfly
 
@@ -237,28 +278,11 @@ Usage: dragonfly [FLAGS]
   CHECK_GT(GetFlag(FLAGS_port), 0u);
   mi_stats_reset();
 
-  base::sys::KernelVersion kver;
-  base::sys::GetKernelVersion(&kver);
-
-  if (kver.kernel < 5 || (kver.kernel == 5 && kver.major < 10)) {
-    LOG(ERROR) << "Kernel 5.10 or later is supported. Exiting...";
-    return 1;
-  }
-
-  int iouring_res = io_uring_queue_init_params(0, nullptr, nullptr);
-  if (-iouring_res == ENOSYS) {
-    LOG(ERROR) << "iouring system call interface is not supported. Exiting...";
-    return 1;
-  }
-
   if (GetFlag(FLAGS_dbnum) > dfly::kMaxDbId) {
     LOG(ERROR) << "dbnum is too big. Exiting...";
     return 1;
   }
 
-  CHECK_LT(kver.major, 99u);
-  dfly::kernel_version = kver.kernel * 100 + kver.major;
-
   string pidfile_path = GetFlag(FLAGS_pidfile);
   if (!pidfile_path.empty()) {
     if (!CreatePidFile(pidfile_path)) {
@@ -287,14 +311,28 @@ Usage: dragonfly [FLAGS]
   mi_option_enable(mi_option_show_errors);
   mi_option_set(mi_option_max_warnings, 0);
 
-  uring::UringPool pp{1024};
+  base::sys::KernelVersion kver;
-  pp.Run();
+  base::sys::GetKernelVersion(&kver);
 
-  AcceptServer acceptor(&pp);
+  CHECK_LT(kver.major, 99u);
+  dfly::kernel_version = kver.kernel * 100 + kver.major;
 
-  int res = dfly::RunEngine(&pp, &acceptor) ? 0 : -1;
+  unique_ptr<util::ProactorPool> pool;
 
-  pp.Stop();
+  bool use_epoll = ShouldUseEpollAPI(kver);
+  if (use_epoll) {
+    pool.reset(new epoll::EpollPool);
+  } else {
+    pool.reset(new uring::UringPool(1024));  // 1024 - iouring queue size.
+  }
+
+  pool->Run();
+
+  AcceptServer acceptor(pool.get());
+
+  int res = dfly::RunEngine(pool.get(), &acceptor) ? 0 : -1;
+
+  pool->Stop();
 
   if (!pidfile_path.empty()) {
     unlink(pidfile_path.c_str());

src/server/server_family.cc

@@ -41,6 +41,7 @@ extern "C" {
 #include "server/version.h"
 #include "strings/human_readable.h"
 #include "util/accept_server.h"
+#include "util/fibers/fiber_file.h"
 #include "util/uring/uring_file.h"
 
 using namespace std;
@@ -66,6 +67,7 @@ using absl::StrCat;
 using namespace facade;
 using strings::HumanReadableNumBytes;
 using util::ProactorBase;
+using util::fibers_ext::FiberQueueThreadPool;
 using util::http::StringResponse;
 
 namespace {
@@ -169,7 +171,8 @@ class LinuxWriteWrapper : public io::Sink {
 
 class RdbSnapshot {
  public:
-  RdbSnapshot() {}
+  RdbSnapshot(FiberQueueThreadPool* fq_tp) : fq_tp_(fq_tp) {
+  }
 
   error_code Start(bool single_shard, const std::string& path, const StringVec& lua_scripts);
   void StartInShard(EngineShard* shard);
@@ -187,13 +190,12 @@ class RdbSnapshot {
 
  private:
   bool started_ = false;
-
+  FiberQueueThreadPool* fq_tp_;
   std::unique_ptr<io::Sink> io_sink_;
   std::unique_ptr<RdbSaver> saver_;
   RdbTypeFreqMap freq_map_;
 };
 
-
 io::Result<size_t> LinuxWriteWrapper::WriteSome(const iovec* v, uint32_t len) {
   io::Result<size_t> res = lf_->WriteSome(v, len, offset_, 0);
   if (res) {
@@ -203,15 +205,24 @@ io::Result<size_t> LinuxWriteWrapper::WriteSome(const iovec* v, uint32_t len) {
   return res;
 }
 
-error_code RdbSnapshot::Start(bool sharded_snapshot,
+error_code RdbSnapshot::Start(bool sharded_snapshot, const std::string& path,
-                              const std::string& path, const StringVec& lua_scripts) {
+                              const StringVec& lua_scripts) {
-  auto res = uring::OpenLinux(path, kRdbWriteFlags, 0666);
+  bool is_direct = false;
-  if (!res) {
+  if (fq_tp_) {  // EPOLL
-    return res.error();
+    auto res = util::OpenFiberWriteFile(path, fq_tp_);
+    if (!res)
+      return res.error();
+    io_sink_.reset(*res);
+  } else {
+    auto res = uring::OpenLinux(path, kRdbWriteFlags, 0666);
+    if (!res) {
+      return res.error();
+    }
+    io_sink_.reset(new LinuxWriteWrapper(res->release()));
+    is_direct = kRdbWriteFlags & O_DIRECT;
   }
 
-  io_sink_.reset(new LinuxWriteWrapper(res->release()));
+  saver_.reset(new RdbSaver(io_sink_.get(), sharded_snapshot, is_direct));
-  saver_.reset(new RdbSaver(io_sink_.get(), sharded_snapshot, kRdbWriteFlags & O_DIRECT));
 
   return saver_->SaveHeader(lua_scripts);
 }
@@ -222,6 +233,9 @@ error_code RdbSnapshot::SaveBody() {
 
 error_code RdbSnapshot::Close() {
   // TODO: to solve it in a more elegant way.
+  if (fq_tp_) {
+    return static_cast<io::WriteFile*>(io_sink_.get())->Close();
+  }
   return static_cast<LinuxWriteWrapper*>(io_sink_.get())->Close();
 }
 
@@ -330,6 +344,9 @@ void ServerFamily::Init(util::AcceptServer* acceptor, util::ListenerInterface* m
   dfly_cmd_.reset(new DflyCmd(main_listener, this));
 
   pb_task_ = shard_set->pool()->GetNextProactor();
+  if (pb_task_->GetKind() == ProactorBase::EPOLL) {
+    fq_threadpool_.reset(new FiberQueueThreadPool());
+  }
 
   // Unlike EngineShard::Heartbeat that runs independently in each shard thread,
   // this callback runs in a single thread and it aggregates globally stats from all the shards.
@@ -489,8 +506,14 @@ void ServerFamily::SnapshotScheduling(const SnapshotSpec&& spec) {
 }
 
 error_code ServerFamily::LoadRdb(const std::string& rdb_file) {
-  io::ReadonlyFileOrError res = uring::OpenRead(rdb_file);
   error_code ec;
+  io::ReadonlyFileOrError res;
+
+  if (fq_threadpool_) {
+    res = util::OpenFiberReadFile(rdb_file, fq_threadpool_.get());
+  } else {
+    res = uring::OpenRead(rdb_file);
+  }
 
   if (res) {
     io::FileSource fs(*res);
@@ -777,7 +800,7 @@ error_code ServerFamily::DoSave(bool new_version, Transaction* trans, string* er
       abs_path += shard_file;
 
       VLOG(1) << "Saving to " << abs_path;
-      snapshots[sid].reset(new RdbSnapshot);
+      snapshots[sid].reset(new RdbSnapshot(fq_threadpool_.get()));
       auto& snapshot = snapshots[sid];
       error_code local_ec = snapshot->Start(true, abs_path.generic_string(), lua_scripts);
       if (local_ec) {
@@ -800,7 +823,7 @@ error_code ServerFamily::DoSave(bool new_version, Transaction* trans, string* er
     path += filename;
     VLOG(1) << "Saving to " << path;
 
-    snapshots[0].reset(new RdbSnapshot);
+    snapshots[0].reset(new RdbSnapshot(fq_threadpool_.get()));
     ec = snapshots[0]->Start(false, path.generic_string(), lua_scripts);
 
     if (!ec) {
@@ -1104,12 +1127,15 @@ void ServerFamily::Info(CmdArgList args, ConnectionContext* cntx) {
   Metrics m = GetMetrics();
 
   if (should_enter("SERVER")) {
+    ProactorBase::ProactorKind kind = ProactorBase::me()->GetKind();
+    const char* multiplex_api = (kind == ProactorBase::IOURING) ? "iouring" : "epoll";
+
     ADD_HEADER("# Server");
 
     append("redis_version", GetVersion());
     append("redis_mode", "standalone");
     append("arch_bits", 64);
-    append("multiplexing_api", "iouring");
+    append("multiplexing_api", multiplex_api);
     append("tcp_port", GetFlag(FLAGS_port));
 
     size_t uptime = m.uptime;

src/server/server_family.h

@@ -163,6 +163,7 @@ class ServerFamily {
   std::atomic_bool is_saving_{false};
 
   util::fibers_ext::Done is_snapshot_done_;
+  std::unique_ptr<util::fibers_ext::FiberQueueThreadPool> fq_threadpool_;
 };
 
 }  // namespace dfly