PreCacheFile.cc

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#include "PreCacheFile.hh"
#include "FileOperations.hh"
#include "statp.hh"
#include <cstdio>
#include <sys/types.h>

namespace openmsx {

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()
{
        struct stat st;
        if (stat(name.c_str(), &st)) return;
        if (!S_ISREG(st.st_mode)) {
                // 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 < 1024 * 1024) {
                // only pre-cache small files

                const size_t BLOCK_SIZE = 4096;
                unsigned block = 0;
                unsigned repeat = 0;
                while (true) {
                        if (exitLoop) break;

                        char buf[BLOCK_SIZE];
                        if (fseek(file.get(), block * BLOCK_SIZE, SEEK_SET)) break;
                        size_t read = fread(buf, 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