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Writing to the file on the HD is your massive bottleneck no matter how many threads you throw around. The limit is the physical memory access speed, not processing power. And since it is such a bottleneck, you should have a thread solely focusing on this job, similar to what @dmckee‭ suggested.
Now what you can do is to have the file writer thread work with large chunks of fixed sizes. Don't just write a few lines each time, write a large chunk. You can have other threads preparing the data in advance.
Suppose you have some logging function where you pass on one string at a time, in some icky inconvenient format like `std::string` or some QT class. Instead of writing 5 strings each one at a time, with a length of some 10 to 100 bytes, show these into a raw byte buffer and let it build up to a certain size. Computers love multiples of 8, so maybe work with chunks of 256 or 512 bytes at a time. And yes we are talking about raw C strings here, forget all about "overloading ofstream", "type generic logging" and other such time-consuming fluff.
As a positive little side-effect, these raw chunks will also be very cache-friendly, unlike a bunch of heap allocated fragments from `std::string`/`std::vector` etc. But RAM access speed is a minor concern compared to HD access speed.
> This gets even more nasty in case the writer thread is too slow: It will pile up events consuming virtually all RAM in the system.
Yeah that's the thing with queues: if your real-time specification doesn't add up, so that you never end up with an empty queue, then no amount of queueing will save you. The problem could simply be that you are saving too much data too frequently.
Make sure to benchmark on an old SATA/SCSI HD and not on a SSD.

Writing to the file on the HD is your massive bottleneck no matter how many threads you throw around. The limit is the physical memory access speed, not processing power. And since it is such a bottleneck, you should have a thread solely focusing on this job, similar to what @dmckee‭ suggested.
Now what you can do is to have the file writer thread work with large chunks of fixed sizes. Don't just write a few lines each time, write a large chunk. You can have other threads preparing the data in advance.
Suppose you have some logging function where you pass on one string at a time, in some icky inconvenient format like `std::string` or some Qt class. Instead of writing 5 strings each one at a time, with a length of some 10 to 100 bytes, show these into a raw byte buffer and let it build up to a certain size. Computers love multiples of 8, so maybe work with chunks of 256 or 512 bytes at a time. And yes we are talking about raw C strings here, forget all about "overloading ofstream", "type generic logging" and other such time-consuming fluff.
As a positive little side-effect, these raw chunks will also be very cache-friendly, unlike a bunch of heap allocated fragments from `std::string`/`std::vector` etc. But RAM access speed is a minor concern compared to HD access speed.
> This gets even more nasty in case the writer thread is too slow: It will pile up events consuming virtually all RAM in the system.
Yeah that's the thing with queues: if your real-time specification doesn't add up, so that you never end up with an empty queue, then no amount of queueing will save you. The problem could simply be that you are saving too much data too frequently.
Make sure to benchmark on an old SATA/SCSI HD and not on a SSD.

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