linux-kernel - Re: [V9fs-developer] [PATCH v2 4/6] 9p: Embed wait_queue_head into p9_req

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Message-ID: <20180712165914.7d1f949f@bahia.lan>
Date:   Thu, 12 Jul 2018 16:59:14 +0200
From:   Greg Kurz <groug@...d.org>
To:     Dominique Martinet <asmadeus@...ewreck.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Latchesar Ionkov <lucho@...kov.net>,
        Eric Van Hensbergen <ericvh@...il.com>,
        linux-kernel@...r.kernel.org, Ron Minnich <rminnich@...dia.gov>,
        linux-fsdevel@...r.kernel.org, v9fs-developer@...ts.sourceforge.net
Subject: Re: [V9fs-developer] [PATCH v2 4/6] 9p: Embed wait_queue_head into
 p9_req_t

On Thu, 12 Jul 2018 16:40:12 +0200
Dominique Martinet <asmadeus@...ewreck.org> wrote:

> Greg Kurz wrote on Thu, Jul 12, 2018:
> > This is true when all tags have been used at least once. But the current code
> > lazily allocates the wait_queue_head_t, ie, only when a tag is used for the
> > first time. Your patch causes a full row of wait_quest_head_t to be pre-allocated.
> > 
> > ie, P9_ROW_MAXTAG * 24 = 255 * 24 = 6120
> > 
> > instead of (P9_ROW_MAXTAG * 8) + 24 = 255 * 8 + 24 = 2064
> > 
> > This is nearly a page of allocated memory that might be never used.
> > 
> > Not sure if this is a problem though...  
> 
> I thought the exact same, but the next patch in the serie removes that
> array and allocates even more lazily with a slab, so this was a
> no-brainer :)
> 

Ah ? I haven't started looking at that *big* other patch yet... :)