Re: [PATCH] readhistfile: avoid thousands of lseek(2) syscalls via ftell()

[Mikael Magnusson <mikachu@gmail.com>]

On 2/7/20, Michael Stapelberg <michael+zsh@stapelberg.ch> wrote:
> [please cc me in replies, as I’m not subscribed to this list]
>
> Before this change, zsh startup time was dominated by lseek(2) system calls
> on
> the history file, as shown by strace -c:
>
> time     seconds  usecs/call     calls    errors syscall
> ------ ----------- ----------- --------- --------- ----------------
>  97,35    1,112890           1    697153         1 lseek
>   0,99    0,011314           2      5277           read
> […]
>
> This change keeps track of read bytes and the position within the file,
> removing
> all of these system calls.
>
> I verified correctness of the change by comparing fpos with ftell(in) in
> every
> loop iteration.

> +	*readbytes += strlen(buf + start);
>  	int len = start + strlen(buf + start);

int len = strlen(buf + start);
*readbytes += len;
len += start;

Even if you're convinced the compiler will optimize the double strlen
calls, the declaration has to come before code (I don't think we
require new enough C to not require this yet?).

-- 
Mikael Magnusson