Re: [PATCH 01/10] pkt-line: use stack rather than static buffer in packet_write_gently()

From: Jeff King <peff@peff.net>
Date: Jan 13, 2021, 13:29 UTC
Message-ID: <X/71qByO5jSceIFn@coredump.intra.peff.net>
In-Reply-To: <1155a45cf64afb237204429cd4ff2e74f5f7602a.1610465492.git.gitgitgadget@gmail.com>

On Tue, Jan 12, 2021 at 03:31:23PM +0000, Jeff Hostetler via GitGitGadget wrote:

> Teach packet_write_gently() to use a stack buffer rather than a static
> buffer when composing the packet line message.  This helps get us ready
> for threaded operations.

Sounds like a good goal, but...

Show 5 quoted lines

>  static int packet_write_gently(const int fd_out, const char *buf, size_t size)
>  {
> -	static char packet_write_buffer[LARGE_PACKET_MAX];
> +	char packet_write_buffer[LARGE_PACKET_MAX];
>  	size_t packet_size;

64k is awfully big for the stack, especially if you are thinking about having threads. I know we've run into issues around that size before (though I don't offhand recall whether there was any recursion involved).

We might need to use thread-local storage here. Heap would also obviously work, but I don't think we'd want a new allocation per write (or maybe it wouldn't matter; we're making a syscall, so a malloc() may not be that big a deal in terms of performance).

-Peff