I have a historical question. Why do we write and maintain our own code to generate the metrics response instead of using the prometheus client library?
> I have learned that the /metrics endpoint will be requested by more than > one metrics collect system. In practice, when does this happen? > PrometheusMetricsGenerator#generate will be invoked once in a period(such > as 1 minute), the result will be cached and returned for every metrics > collect request in the period directly. Since there are tradeoffs to the cache duration, we should make the period configurable. Thanks, Michael On Wed, Feb 23, 2022 at 11:06 AM Jiuming Tao <jm...@streamnative.io.invalid> wrote: > > Hi all, > > > > 2. When there are hundreds MB metrics data collected, it causes high heap > > memory usage, high CPU usage and GC pressure. In the > > `PrometheusMetricsGenerator#generate` method, it uses > > `ByteBufAllocator.DEFAULT.heapBuffer()` to allocate memory for writing > > metrics data. The default size of `ByteBufAllocator.DEFAULT.heapBuffer()` > > is 256 bytes, when the buffer resizes, the new buffer capacity is 512 > > bytes(power of 2) and with `mem_copy` operation. > > If I want to write 100 MB data to the buffer, the current buffer size is > > 128 MB, and the total memory usage is close to 256 MB (256bytes + 512 bytes > > + 1k + .... + 64MB + 128MB). When the buffer size is greater than netty > > buffer chunkSize(16 MB), it will be allocated as UnpooledHeapByteBuf in the > > heap. After writing metrics data into the buffer, return it to the client > > by jetty, jetty will copy it into jetty's buffer with memory allocation in > > the heap, again! > > In this condition, for the purpose of saving memory, avoid high CPU > > usage(too much memory allocations and `mem_copy` operations) and reducing > > GC pressure, I want to change `ByteBufAllocator.DEFAULT.heapBuffer()` to > > `ByteBufAllocator.DEFAULT.compositeDirectBuffer()`, it wouldn't cause > > `mem_copy` operations and huge memory allocations(CompositeDirectByteBuf is > > a bit slowly in read/write, but it's worth). After writing data, I will > > call the `HttpOutput#write(ByteBuffer)` method and write it to the client, > > the method won't cause `mem_copy` (I have to wrap ByteBuf to ByteBuffer, if > > ByteBuf wrapped, there will be zero-copy). > > The jdk in my local is jdk15, I just noticed that in jdk8, ByteBuffer cannot > be extended and implemented. So, if allowed, I will write metrics data to > temp files and send it to client by jetty’s send_file. It will be turned out > a better performance than `CompositeByteBuf`, and takes lower CPU usage due > to I/O blocking.(The /metrics endpoint will be a bit slowly, I believe it’s > worth). > If not allowed, it’s no matter and it also has a better performance than > `ByteBufAllocator.DEFAULT.heapBuffer()`(see the first image in original mail). > > Thanks, > Tao Jiuming
