comparison rust/hg-core/src/revlog/mod.rs @ 52181:3d797007905d

rust: populate mmaps in a separate thread if possible

Same rationale as b619ba39d10a.

52180:1032bb0ef365

    }
}

type IndexData = Box<dyn Deref<Target = [u8]> + Send + Sync>;

/// Open the revlog [`Index`] at `index_path`, through the `store_vfs` and the
/// given `options`. This controls whether (and how) we `mmap` the index file,
/// and returns an empty buffer if the index does not exist on disk.
/// This is only used when doing pure-Rust work, in Python contexts this is
/// unused at the time of writing.

            {
                let size = store_vfs.file_size(&file)?;
                if size >= threshold {
                    // TODO madvise populate read in a background thread
                    let mut mmap_options = MmapOptions::new();
                    // This does nothing on platforms where it's not defined
                    mmap_options.populate();
                    // Safety is "enforced" by locks and assuming other
                    // processes are well-behaved. If any misbehaving or
                    // malicious process does touch the index, it could lead
                    // to corruption. This is somewhat inherent to file-based
                    // `mmap`, though some platforms have some ways of
                    // mitigating.
                    // TODO linux: set the immutable flag with `chattr(1)`?
                    let mmap = unsafe { mmap_options.map(&file) }
                        .when_reading_file(index_path)?;
                    Some(Box::new(mmap) as IndexData)
                } else {
                    None
                }
            } else {

52181:3d797007905d

    }
}

type IndexData = Box<dyn Deref<Target = [u8]> + Send + Sync>;

/// TODO We should check for version 5.14+ at runtime, but we either should
/// add the `nix` dependency to get it efficiently, or vendor the code to read
/// both of which are overkill for such a feature. If we need this dependency
/// for more things later, we'll use it here too.
#[cfg(target_os = "linux")]
fn can_advise_populate_read() -> bool {
    true
}

#[cfg(not(target_os = "linux"))]
fn can_advise_populate_read() -> bool {
    false
}

/// Call `madvise` on the mmap with `MADV_POPULATE_READ` in a separate thread
/// to populate the mmap in the background for a small perf improvement.
#[cfg(target_os = "linux")]
fn advise_populate_read_mmap(mmap: &memmap2::Mmap) {
    const MADV_POPULATE_READ: i32 = 22;

    // This is fine because the mmap is still referenced for at least
    // the duration of this function, and the kernel will reject any wrong
    // address.
    let ptr = mmap.as_ptr() as u64;
    let len = mmap.len();

    // Fire and forget. The `JoinHandle` returned by `spawn` is dropped right
    // after the call, the thread is thus detached. We don't care about success
    // or failure here.
    std::thread::spawn(move || unsafe {
        // mmap's pointer is always page-aligned on Linux. In the case of
        // file-based mmap (which is our use-case), the length should be
        // correct. If not, it's not a safety concern as the kernel will just
        // ignore unmapped pages and return ENOMEM, which we will promptly
        // ignore, because we don't care about any errors.
        libc::madvise(ptr as *mut libc::c_void, len, MADV_POPULATE_READ);
    });
}

#[cfg(not(target_os = "linux"))]
fn advise_populate_read_mmap(mmap: &memmap2::Mmap) {}

/// Open the revlog [`Index`] at `index_path`, through the `store_vfs` and the
/// given `options`. This controls whether (and how) we `mmap` the index file,
/// and returns an empty buffer if the index does not exist on disk.
/// This is only used when doing pure-Rust work, in Python contexts this is
/// unused at the time of writing.

            {
                let size = store_vfs.file_size(&file)?;
                if size >= threshold {
                    // TODO madvise populate read in a background thread
                    let mut mmap_options = MmapOptions::new();
                    if !can_advise_populate_read() {
                        // Fall back to populating in the main thread if
                        // post-creation advice is not supported.
                        // Does nothing on platforms where it's not defined.
                        mmap_options.populate();
                    }
                    // Safety is "enforced" by locks and assuming other
                    // processes are well-behaved. If any misbehaving or
                    // malicious process does touch the index, it could lead
                    // to corruption. This is somewhat inherent to file-based
                    // `mmap`, though some platforms have some ways of
                    // mitigating.
                    // TODO linux: set the immutable flag with `chattr(1)`?
                    let mmap = unsafe { mmap_options.map(&file) }
                        .when_reading_file(index_path)?;

                    if can_advise_populate_read() {
                        advise_populate_read_mmap(&mmap);
                    }

                    Some(Box::new(mmap) as IndexData)
                } else {
                    None
                }
            } else {