[io] add RRawFile::DiscourageReadAheadImpl()

io/io/inc/ROOT/RRawFile.hxx

@@ -140,6 +140,9 @@ protected:
    /// By default implemented as a loop of ReadAt calls but can be overwritten, e.g. XRootD or DAVIX implementations
    virtual void ReadVImpl(RIOVec *ioVec, unsigned int nReq);
 
+   // Default implementation: no-op
+   virtual void SetDiscourageReadAheadImpl(bool /* value */) {}
+
    /// Open the file if not already open. Otherwise noop.
    void EnsureOpen();
 
@@ -178,8 +181,9 @@ public:
    /// Returns the limits regarding the ioVec input to ReadV for this specific file; may open the file as a side-effect.
    virtual RIOVecLimits GetReadVLimits() { return RIOVecLimits(); }
 
-   /// Turn off buffered reads; all scalar read requests go directly to the implementation. Buffering can be turned
-   /// back on.
+   /// Turn on/off buffered reads; if off, all scalar read requests go directly to the implementation. Buffering also
+   /// turns on and off OS read-ahead where supported: if buffering is switched, SetDiscourageReadAheadImpl() will be
+   /// called accordingly.
    void SetBuffering(bool value);
    bool IsBuffering() const { return fIsBuffering; }
 

io/io/inc/ROOT/RRawFileUnix.hxx

@@ -37,6 +37,7 @@ protected:
    size_t ReadAtImpl(void *buffer, size_t nbytes, std::uint64_t offset) final;
    void ReadVImpl(RIOVec *ioVec, unsigned int nReq) final;
    std::uint64_t GetSizeImpl() final;
+   void SetDiscourageReadAheadImpl(bool value) final;
 
 public:
    RRawFileUnix(std::string_view url, RRawFile::ROptions options);

io/io/src/RRawFile.cxx

@@ -92,6 +92,7 @@ void ROOT::Internal::RRawFile::EnsureOpen()
 
    OpenImpl();
    fIsOpen = true;
+   SetDiscourageReadAheadImpl(!fIsBuffering);
 }
 
 void ROOT::Internal::RRawFile::ReadVImpl(RIOVec *ioVec, unsigned int nReq)
@@ -200,6 +201,8 @@ void ROOT::Internal::RRawFile::SetBuffering(bool value)
    fIsBuffering = value;
    if (!fIsBuffering)
       fBufferSpace.reset();
+   if (fIsOpen)
+      SetDiscourageReadAheadImpl(!fIsBuffering);
 }
 
 bool ROOT::Internal::RRawFile::Readln(std::string &line)

io/io/src/RRawFileUnix.cxx

@@ -92,6 +92,15 @@ void ROOT::Internal::RRawFileUnix::OpenImpl()
    }
 }
 
+void ROOT::Internal::RRawFileUnix::SetDiscourageReadAheadImpl(bool value)
+{
+#ifndef __APPLE__
+   posix_fadvise(fFileDes, 0, 0, value ? POSIX_FADV_RANDOM : POSIX_FADV_SEQUENTIAL);
+#else
+   (void)value;
+#endif
+}
+
 void ROOT::Internal::RRawFileUnix::ReadVImpl(RIOVec *ioVec, unsigned int nReq)
 {
 #ifdef R__HAS_URING

io/io/test/RRawFile.cxx

@@ -15,6 +15,7 @@ class RRawFileMock : public RRawFile {
 public:
    std::string fContent;
    unsigned fNumReadAt;
+   bool fDiscourageReadAhead = false;
 
    RRawFileMock(const std::string &content, RRawFile::ROptions options)
      : RRawFile("", options), fContent(content), fNumReadAt(0) { }
@@ -39,6 +40,8 @@ class RRawFileMock : public RRawFile {
    }
 
    std::uint64_t GetSizeImpl() final { return fContent.size(); }
+
+   void SetDiscourageReadAheadImpl(bool val) final { fDiscourageReadAhead = val; }
 };
 
 } // anonymous namespace
@@ -272,3 +275,22 @@ TEST(RRawFileTFile, TFile)
    EXPECT_EQ(seek[2], 0);
    EXPECT_EQ(seek[3], 100);
 }
+
+TEST(RRawFile, Readahead)
+{
+   {
+      RRawFileMock mock("", ROOT::Internal::RRawFile::ROptions());
+      (void)mock.GetSize();
+      EXPECT_FALSE(mock.fDiscourageReadAhead);
+   }
+
+   {
+      RRawFileMock mock("", ROOT::Internal::RRawFile::ROptions());
+      mock.SetBuffering(false);
+      EXPECT_FALSE(mock.fDiscourageReadAhead);
+      (void)mock.GetSize();
+      EXPECT_TRUE(mock.fDiscourageReadAhead);
+      mock.SetBuffering(true);
+      EXPECT_FALSE(mock.fDiscourageReadAhead);
+   }
+}