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chore: Reduce memory consumption when migrating huge values #4119

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chore: Reduce memory consumption when migrating huge values #4119

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51 changes: 38 additions & 13 deletions src/server/rdb_save.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -933,14 +933,13 @@ VersionBuffer MakeRdbVersion() {
return buf;
}

CrcBuffer MakeCheckSum(std::string_view dump_res) {
uint64_t chksum = crc64(0, reinterpret_cast<const uint8_t*>(dump_res.data()), dump_res.size());
CrcBuffer WrapCheckSum(uint64_t crc) {
CrcBuffer buf;
absl::little_endian::Store64(buf.data(), chksum);
absl::little_endian::Store64(buf.data(), crc);
return buf;
}

void AppendFooter(io::StringSink* dump_res) {
void AppendFooter(io::Sink* dump_res, uint64_t crc) {
auto to_bytes = [](const auto& buf) {
return io::Bytes(reinterpret_cast<const uint8_t*>(buf.data()), buf.size());
};
Expand All @@ -952,18 +951,45 @@ void AppendFooter(io::StringSink* dump_res) {
* RDB version and CRC are both in little endian.
*/
const auto ver = MakeRdbVersion();
dump_res->Write(to_bytes(ver));
const auto crc = MakeCheckSum(dump_res->str());
dump_res->Write(to_bytes(crc));
auto ver_bytes = to_bytes(ver);
dump_res->Write(ver_bytes);

crc = crc64(crc, ver_bytes.data(), ver_bytes.size());
const auto crc_buf = WrapCheckSum(crc);
dump_res->Write(to_bytes(crc_buf));
}
} // namespace

void SerializerBase::DumpObject(const CompactObj& obj, io::StringSink* out) {
void SerializerBase::DumpObject(const CompactObj& obj, io::Sink* out) {
CompressionMode compression_mode = GetDefaultCompressionMode();
if (compression_mode != CompressionMode::NONE) {
compression_mode = CompressionMode::SINGLE_ENTRY;
}
RdbSerializer serializer(compression_mode);
uint64_t crc = 0;
uint64_t total_size = 0;
auto flush = [&](RdbSerializer* srz) {
io::StringSink s;

Comment on lines +971 to +972
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Do want to flush directly to a string and then to the sink ? Is there a way to do it directly ?

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the reason I do it as such is to be able to compute the CRC value for the additional data, so we can flush to the sink directly (otherwise we can't compute the crc)

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(otherwise we can't compute the crc)

Yep that answers it 😄

// Use kFlushMidEntry because we append a footer below
auto ec = srz->FlushToSink(&s, SerializerBase::FlushState::kFlushMidEntry);
RETURN_ON_ERR(ec);

string_view sv = s.str();
if (sv.empty()) {
return ec; // Nothing to do
}

total_size += sv.size();
crc = crc64(crc, reinterpret_cast<const uint8_t*>(sv.data()), sv.size());
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Nice you can accumulate crc based on the previous value + the new serialized

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🤓


return out->Write(io::Buffer(sv));
};

RdbSerializer serializer(compression_mode, [&](size_t size, SerializerBase::FlushState) {
auto size_before = total_size;
flush(&serializer);
DCHECK_EQ(size, total_size - size_before);
});
Comment on lines +988 to +992
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Do we want to flush unconditionally on large values ? (even when big value serialization is off ?)

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Nice catch! I was under the (false) assumption that this is called only above serialization_max_chunk_size threshold. I'll modify this area.


// According to Redis code we need to
// 1. Save the value itself - without the key
Expand All @@ -974,10 +1000,9 @@ void SerializerBase::DumpObject(const CompactObj& obj, io::StringSink* out) {
CHECK(!ec);
ec = serializer.SaveValue(obj);
CHECK(!ec); // make sure that fully was successful
ec = serializer.FlushToSink(out, SerializerBase::FlushState::kFlushMidEntry);
CHECK(!ec); // make sure that fully was successful
AppendFooter(out); // version and crc
CHECK_GT(out->str().size(), 10u);
ec = flush(&serializer);
CHECK(!ec); // make sure that fully was successful
AppendFooter(out, crc); // version and crc
}

size_t SerializerBase::SerializedLen() const {
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2 changes: 1 addition & 1 deletion src/server/rdb_save.h
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Original file line number Diff line number Diff line change
Expand Up @@ -156,7 +156,7 @@ class SerializerBase {
virtual ~SerializerBase() = default;

// Dumps `obj` in DUMP command format into `out`. Uses default compression mode.
static void DumpObject(const CompactObj& obj, io::StringSink* out);
static void DumpObject(const CompactObj& obj, io::Sink* out);

// Internal buffer size. Might shrink after flush due to compression.
size_t SerializedLen() const;
Expand Down
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