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kernel: limit fscache to 256MB

Reclaiming pages takes too long when
we sink all our idle pages into
the fscache, yielding to lockloop prints
during relcaim.

Limit the size of the fscache to 256MB
to have more predictable behaviour.

--- a/sys/src/9/port/cache.c

+++ b/sys/src/9/port/cache.c

@@ -9,10 +9,12 @@

 {

 	NHASH		= 128,

 	NFILE		= 4093,		/* should be prime */

-	MAXCACHE	= 8*1024*1024,

-

+	MAXCACHE	= 8*MB,		/* per file */

 	MAPBITS		= 8*sizeof(ulong),

 	NBITMAP		= (PGROUND(MAXCACHE)/BY2PG + MAPBITS-1) / MAPBITS,

+

+	/* maximum number pages in fscache image */

+	TOTALPAGES	= (256*MB)/BY2PG,

 };

 /* devmnt.c: parallel read ahread implementation */

@@ -425,6 +427,8 @@

 				invalidate(m, offset + pn*BY2PG, len);

 				break;

 			}

+			if(fscache.pgref > TOTALPAGES)

+				pagereclaim(&fscache);

 			p = newpage(0, nil, pn*BY2PG);

 			p->daddr = cacheaddr(m, pn);

 			cachedel(&fscache, p->daddr);

-- 

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