Re: [PATCH] ipc: replace costly bailout check in sysvipc_find_ipc()

[Davidlohr Bueso <dbueso@suse.de>]

On 2021-08-09 13:35, Rafael Aquini wrote:
> sysvipc_find_ipc() was left with a costly way to check if the offset
> position fed to it is bigger than the total number of IPC IDs in use.
> So much so that the time it takes to iterate over /proc/sysvipc/* files
> grows exponentially for a custom benchmark that creates "N" SYSV shm
> segments and then times the read of /proc/sysvipc/shm (milliseconds):
> 
>     12 msecs to read   1024 segs from /proc/sysvipc/shm
>     18 msecs to read   2048 segs from /proc/sysvipc/shm
>     65 msecs to read   4096 segs from /proc/sysvipc/shm
>    325 msecs to read   8192 segs from /proc/sysvipc/shm
>   1303 msecs to read  16384 segs from /proc/sysvipc/shm
>   5182 msecs to read  32768 segs from /proc/sysvipc/shm
> 
> The root problem lies with the loop that computes the total amount of 
> ids
> in use to check if the "pos" feeded to sysvipc_find_ipc() grew bigger 
> than
> "ids->in_use". That is a quite inneficient way to get to the maximum 
> index
> in the id lookup table, specially when that value is already provided 
> by
> struct ipc_ids.max_idx.
> 
> This patch follows up on the optimization introduced via commit 
> 15df03c879836
> ("sysvipc: make get_maxid O(1) again") and gets rid of the 
> aforementioned
> costly loop replacing it by a simpler checkpoint based on 
> ipc_get_maxidx()
> returned value, which allows for a smooth linear increase in time 
> complexity
> for the same custom benchmark:
> 
>      2 msecs to read   1024 segs from /proc/sysvipc/shm
>      2 msecs to read   2048 segs from /proc/sysvipc/shm
>      4 msecs to read   4096 segs from /proc/sysvipc/shm
>      9 msecs to read   8192 segs from /proc/sysvipc/shm
>     19 msecs to read  16384 segs from /proc/sysvipc/shm
>     39 msecs to read  32768 segs from /proc/sysvipc/shm
> 
> Signed-off-by: Rafael Aquini <aquini@redhat.com>

Acked-by: Davidlohr Bueso <dbueso@suse.de>