Re: [asio-users] Emulating non-blocking file-IO with asio

[Matt G. <mat...@jc...>]

First, you don't want to dispatch blocking operations, since dispatching one handler from another will directly execute (and therefore block) the former.

Second, you can add a strand per file, but:
1. That only increases concurrency if you've also got more threads.
2. I thought your original use of strands was to keep a thread free to respond to incoming messages.

You could increase the concurrency of your file operations, if you maintain fewer file strands than the total number of worker threads.  Of course, you'll also want to ensure that all operations on a given file are funneled through the same strand.  So, a better approach might be to have two io_services - one for messages and one for file I/O.  Then, in the file I/O service, you can have as many strands as you like.


Matt


-----Original Message-----
From: Thomas Ilsche [mailto:tho...@tu...] 
Sent: Friday, January 18, 2019 12:13
To: asi...@li...
Subject: [asio-users] Emulating non-blocking file-IO with asio

I am writing a service with asio that needs to do file-IO, including opening files, to answer certain messages. The protocol (AMQP /w TSL) is full-duplex and requires regular heartbeats. The files may be on network filesystems and it may be many. So there are cases where doing too much file-IO causes the service to miss heartbeats.

Most unfortunately there seems to be no way to do non-blocking file-IO on Linux, especially opening files. So my idea is

run an io_context with concurrency_hint == 2 do all general protocol (never blocking) stuff in one strand encapsulate all blocking file-IO and post it to one other strand.

My hope is then that there can never be more than one file-io function blocking the io_context so that there is always a thread free to handle the rest.

Is this a good idea? There doesn't seem to be much of a guarantee, particularly given that it is only a concurrency_*hint*. Does it make a difference whether i post/dispatch?

Are there any, more idiomatic, alternatives?

How could I exploit more parallelism for the file-IO when I could create an arbitrary number of strands - basically one per file?

Thanks,
Thomas


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