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92 lines
3.1 KiB
C++
92 lines
3.1 KiB
C++
#pragma once
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#include <Core/Types.h>
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#include <optional>
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#include <memory>
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#include <future>
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#include <boost/noncopyable.hpp>
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#include <Common/Stopwatch.h>
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#include <Common/Priority.h>
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namespace DB
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{
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/** Interface for asynchronous reads from file descriptors.
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* It can abstract Linux AIO, io_uring or normal reads from separate thread pool,
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* and also reads from non-local filesystems.
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* The implementation not necessarily to be efficient for large number of small requests,
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* instead it should be ok for moderate number of sufficiently large requests
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* (e.g. read 1 MB of data 50 000 times per seconds; BTW this is normal performance for reading from page cache).
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* For example, this interface may not suffice if you want to serve 10 000 000 of 4 KiB requests per second.
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* This interface is fairly limited.
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*/
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class IAsynchronousReader : private boost::noncopyable
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{
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public:
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/// For local filesystems, the file descriptor is simply integer
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/// but it can be arbitrary opaque object for remote filesystems.
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struct IFileDescriptor
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{
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virtual ~IFileDescriptor() = default;
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};
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using FileDescriptorPtr = std::shared_ptr<IFileDescriptor>;
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struct LocalFileDescriptor : public IFileDescriptor
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{
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explicit LocalFileDescriptor(int fd_) : fd(fd_) {}
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int fd;
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};
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/// Read from file descriptor at specified offset up to size bytes into buf.
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/// Some implementations may require alignment and it is responsibility of
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/// the caller to provide conforming requests.
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struct Request
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{
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FileDescriptorPtr descriptor;
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size_t offset = 0;
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size_t size = 0;
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char * buf = nullptr;
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Priority priority;
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size_t ignore = 0;
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};
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struct Result
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{
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/// The read data is at [buf + offset, buf + size), where `buf` is from Request struct.
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/// (Notice that `offset` is included in `size`.)
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/// size
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/// Less than requested amount of data can be returned.
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/// If size is zero - the file has ended.
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/// (for example, EINTR must be handled by implementation automatically)
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size_t size = 0;
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/// offset
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/// Optional. Useful when implementation needs to do ignore().
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size_t offset = 0;
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std::unique_ptr<Stopwatch> execution_watch = {};
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explicit operator std::tuple<size_t &, size_t &>() { return {size, offset}; }
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};
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/// Submit request and obtain a handle. This method don't perform any waits.
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/// If this method did not throw, the caller must wait for the result with 'wait' method
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/// or destroy the whole reader before destroying the buffer for request.
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/// The method can be called concurrently from multiple threads.
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virtual std::future<Result> submit(Request request) = 0;
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virtual Result execute(Request request) = 0;
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virtual void wait() = 0;
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/// Destructor must wait for all not completed request and ignore the results.
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/// It may also cancel the requests.
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virtual ~IAsynchronousReader() = default;
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};
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using AsynchronousReaderPtr = std::shared_ptr<IAsynchronousReader>;
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}
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