PreCacheFile.cc

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#include "PreCacheFile.hh"
#include "FileOperations.hh"
#include "narrow.hh"
#include <array>
#include <cstdio>
#include <sys/types.h>
 
namespace openmsx {
 
// TODO when available use:
//     posix_fadvise(int fd, off_t offset, off_t len, int advise)
//       with advise = POSIX_FADV_WILLNEED
 
PreCacheFile::PreCacheFile(std::string name_)
        : name(std::move(name_)), exitLoop(false)
{
        thread = std::thread([this]() { run(); });
}
 
PreCacheFile::~PreCacheFile()
{
        exitLoop = true;
        thread.join();
}
 
void PreCacheFile::run()
{
        if (!FileOperations::isRegularFile(name)) {
                // don't pre-cache non regular files (e.g. /dev/fd0)
                return;
        }
 
        auto file = FileOperations::openFile(name, "rb");
        if (!file) return;
 
        fseek(file.get(), 0, SEEK_END);
        auto size = ftell(file.get());
        if (size < 1024l * 1024l) {
                // only pre-cache small files
 
                const size_t BLOCK_SIZE = 4096;
                unsigned block = 0;
                unsigned repeat = 0;
                while (true) {
                        if (exitLoop) break;
 
                        std::array<char, BLOCK_SIZE> buf;
                        if (fseek(file.get(), narrow_cast<long>(block * BLOCK_SIZE), SEEK_SET)) break;
                        size_t read = fread(buf.data(), 1, BLOCK_SIZE, file.get());
                        if (read != BLOCK_SIZE) {
                                // error or end-of-file reached,
                                // in both cases stop pre-caching
                                break;
                        }
 
                        // Just reading a file linearly from front to back
                        // makes Linux classify the read as a 'streaming read'.
                        // Linux doesn't cache those. To avoid this we read
                        // some of the blocks twice.
                        if (repeat != 0) {
                                --repeat; ++block;
                        } else {
                                repeat = 5;
                        }
                }
        }
}
 
} // namespace openmsx