linux-kernel - Re: [iov_iter] 9bd0e337c6: will-it-scale.per_process

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Message-ID: <98237.1607082636@warthog.procyon.org.uk>
Date:   Fri, 04 Dec 2020 11:50:36 +0000
From:   David Howells <dhowells@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     dhowells@...hat.com, kernel test robot <oliver.sang@...el.com>,
        lkp@...ts.01.org, kernel test robot <lkp@...el.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Feng Tang <feng.tang@...el.com>, zhengjun.xing@...el.com,
        Pavel Begunkov <asml.silence@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [iov_iter] 9bd0e337c6: will-it-scale.per_process_ops -4.8% regression

Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> > FYI, we noticed a -4.8% regression of will-it-scale.per_process_ops due to commit:
> 
> Ok, I guess that's bigger than expected, 

Note that it appears to be testing just the first patch and not the whole
series:

| commit: 9bd0e337c633aed3e8ec3c7397b7ae0b8436f163 ("[PATCH 01/29] iov_iter: Switch to using a table of operations")

that just adds an indirection table without taking away any of the conditional
branching.  It seems quite likely, though, that even if you add all the other
patches, you won't get back enough to make it worth it.

David