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From: lars@myab.UUCP (lars)
Newsgroups: comp.unix.wizards
Subject: Re: bflush algorithm
Message-ID: <151@myab.UUCP>
Date: Fri, 7-Nov-86 05:36:12 EST
Article-I.D.: myab.151
Posted: Fri Nov  7 05:36:12 1986
Date-Received: Mon, 10-Nov-86 06:15:03 EST
References: <244@miduet.gec-mi-at.co.uk>
Reply-To: lars@myab.UUCP (lars)
Organization: Myab Gothenburg, Sweden
Lines: 32
Keywords: bflush

In article <244@miduet.gec-mi-at.co.uk> jgh@gec-mi-at.co.uk (Jeremy Harris) writes:
>After bflush finds a delayed-write block and asynch-writes it, it scans again
>from the start of the freelist
>
>I assume this is for safety in case some other process inserts a delayed-write
>block on the freelist, but who will do this?
...
>o	The  disk driver  strategy  routine called by bwrite?   I see no
>	reason for it to manipulate the freelist.
...
>Jeremy Harris	jgh@gec-mi-at.co.uk	...!mcvax!ukc!hrc63!miduet!jgh

In fact, strategy usually manipulates the free list, or at least
the free list pointers of its bp. These pointers are used for
maintaining an internal (to strategy) sorted list of disk operations.

This implies that bp->av_forw is not valid after a call to
strategy with bp. This is easily solved with an extra variabel
which "saves" the forward link before the call to strategy.

We implemented this scheme, plus an extra test:
After the whole free list was taken care of, the whole procedure
was restarted if and only if any buffer really was "flushed".

System performance on a sync was much improved.

By the way, we use 2000 buffers (UNIX System V.2) with 8 Mbytes of RAM.
What is the optimal number of buffers regarded to size of memory ?
-- 
______________________________________________________
    Lars Pensjo
    {decvax,philabs}!mcvax!enea!chalmers!myab!lars